[Linter v2] Upgrade to new Linter API

[ymtszw]

Attempts to resolve https://github.com/halohalospecial/atom-elmjutsu/issues/153

Since Linter 2.3.0 dropped Linter v1 API support, this patch upgrades Linter provider of Elmjutsu to be compatible with Linter v2 API.

I had tested this with Linter 2.3.0, linter-ui-default 1.7.1, and it seemingly works, but definitely require other people's eyes. Especially, I have not checked every kind of possible messages that can be emitted from elm compiler.

As stated in upgrade instruction:

• type is now severity
• filePath & range combination is now contained in location object
• html is removed and description added

Also, not explicitly stated but:

• text is replaced with excerpt and is now mandatory
• It says description accepts markdown, though subset of HTML is valid in markdown, it can actually handle HTML as well (as long as it is sanitized and not contain prohibited tags, I assume)
  • As long as currently-used JSX formatting and renderToStaticMarkup() produces mostly simple and sanitized HTML, it should work

Other things I found:

• Messages provided from v2 API are, when consumed, shown in collapsed form by default
• We can expand this and reveal marked-up description, by clicking ">" icon to the left, or invoke linter-ui-default:expand-tooltip command
• Tooltip expansion (linter-ui-default:expand-tooltip) is key-bindable. Though it behaves in "expand while holding the key" manner.
  • This is similar to atom-ide-ui I believe? And it is actually nice since it will not block our line-of-sight to actual buffer with large tooltip!
  • Keybind sample is like this:

    'atom-workspace':
      'ctrl': 'linter-ui-default:expand-tooltip'

  • Added this to README.md
• In v2 Messages, there are solutions property for quick fixes, and references property for in-project references.
  • Current Quick Fixes can be also plugged into new solutions property, since it now allows arbitrary function invocation, not just text replacement. Though at a glance, it require some extra work :P
  • It is aesthetically poor in linter-ui-default (plain "FIX" button on tooltip) compared to atom-ide-ui's code actions, though definitely better than nothing!

[Arcanemagus]

text is replaced with excerpt and is now mandatory

The replacement is noted, but it looks like I missed the part where it was now mandatory when writing that.

It says description accepts markdown, though subset of HTML is valid in markdown, it can actually handle HTML as well

Interestingly it looks like the current implementation in linter-ui-default runs text descriptions through marked... but currently isn't sanitizing the result from function descriptions 🤦‍♂️. I wouldn't rely on that behaviour though as that's a bug.

And it is actually nice since it will not block our line-of-sight to actual buffer with large tooltip!

That's exactly why it isn't expanded by default 😉.