Adding event type strings to use with `AddEventListener`

For documentation and ease of use, event types could be added to aid users with available event types (these are hard to come across in a complete list on the internet)

const (
    EventClick = "click"
    EventLoad = "load"
    EventAbort = "abort"
    // ...
)

Example of what I mean

```go package main // UI events. const ( // The load event fires when the webpage finishes loading. // It can also fire on nodes of elements like images, scripts, or objects. EventLoad = "load" // Fires when a user leaves the page. Usually because they click on a link. EventUnload = "unload" // This event fires when the browser encounters a JavaScript error or an asset that doesn’t exist. EventError = "error" // It fires when we resize the browser window. But browsers repeatedly fire this event, // so avoid using this event to trigger complicated code; it might make the page less responsive. EventResize = "resize" // This event fires when the user scrolls up/down on the browser window. // It can relate to the entire page or a specific element on the page. EventScroll = "scroll" ) // Focus and blur events. const ( // This event fires, for a specific DOM node, when an element gains focus. EventFocus = "focus" // This fires, for a specific DOM node, when an element loses focus. EventBlur = "blur" ) // Human interface device input events. const ( // Mouse events or touchpad // This event fires when the user clicks on the primary mouse button (usually the left button). // This event also fires if the user presses the Enter key on the keyboard when an element has focus. // Touch-screen: A tap on the screen acts like a single primary mouse button click. EventClick = "click" // It fires when the user clicks down on any mouse button. EventMouseDown, EventTouchstart = "mousedown", "touchstart" // We have separate mousedown and mouseup events to add drag-and-drop functionality // or controls in game development. Don’t forget a click event is the combination // of mousedown and mouseup events. // It fires when the user releases a mouse button. EventMouseUp = "mouseup" // It fires when the user moves the cursor, which was inside an element before, outside the element. // We can say that it fires when the cursor moves off the element. EventMouseOut = "mouseout" // It fires when the user moves the cursor, which was outside an element before, inside the element. // We can say that it fires when we move the cursor over the element. EventMouseOver = "mouseover" // It fires when the user moves the cursor around the element. This event is frequently triggered. EventMouseMove = "mousemove" // Keyboard events // The keydown and keypress events fire before a character appears on the screen, // the keyup fires after it shows. To know the key pressed when you use the // keydown and keypress events, the event object has a keyCode property. // This property, instead of returning the letter for that key, returns // the ASCII code of the lowercase for that key. // The keyup event fires when the user releases a key on the keyboard. EventKeyUp = "keyup" // It fires when the user presses any key in the keyboard. // If the user holds down the key, this event fires repeatedly. EventKeyDown = "keydown" // It fires when the user presses a key that results in printing a character on the screen. // This event fires repeatedly if the user holds down the key. // This event will not fire for the enter, tab, or arrow keys; the keydown event would. EventKeyPress = "keypress" // This event fires when the user clicks the primary mouse button, in quick succession, twice. EventDoubleClick = "dblclick" ) // Form events const ( // This event fires on the node representing the

element when a user submits a form. EventSubmit = "submit" // It fires when the status of various form elements change. // This is a better option than using the click event because clicking is not the only way users interact with the form. EventChange = "change" // This event fires when the value of an or a

changes
    // (doesn’t fire for deleting in IE9). You can use keydown as a fallback in older browsers.
    EventInput = "input"
)

// HTML5 events.
const (
    // This event triggers when the DOM tree forms i.e. the script is loading.
    // Scripts start to run before all the resources like images, CSS, and JavaScript loads.
    // You can attach this event either to the window or the document objects.
    EventDOMContentLoaded = "DOMContentLoaded"
    // It fires when the URL hash changes without refreshing the entire window.
    // Hashes (#) link specific parts (known as anchors) within a page.
    //  It works on the window object; the event object contains both the oldURL
    // and the newURL properties holding the URLs before and after the hashchange.
    EventHashChange = "hashchange"
    // This event fires on the window object just before the page unloads.
    // This event should only be helpful for the user,
    // not encouraging them to stay on the page.
    // You can add a dialog box to your event, showing a message alerting the users like their changes are not saved.
    EventBeforeUnload = "beforeunload"
)

```

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dominikh / go-js-dom

Adding event type strings to use with `AddEventListener` #82