Add usage example, fix build for use with bundlers and browser

[JosephusPaye]

This PR adds an example Vite project to test the usage of Keen UI:

• using Vite with Keen UI's default and named exports
• using Vite with Keen UI's lib exports
• using Vite with imports from Keen UI's src/ folder
• using a global script tag

Currently a work in progress, help welcome. Status:

• Using the default and named export from Keen UI works, but has no styles
• Using the lib/ export doesn't work yet
• Using imports from keen-ui/src/ folder doesn't work yet
• Haven't added usage example with a global script tag, but it'll be something like this: script-tag-usage-example.zip from some testing I did recently

Edit: All done thanks to help from @kilobyte2007!

See the usage example README for how to run it.

[kilobyte2007]

Hey @JosephusPaye, Do you need any help with this PR?

[JosephusPaye]

Yes definitely. Feel free to help with this.