Add basic support for JavaScript files

[aranchelk]

• Modification of JS file triggers evaluation of associated .purs files (same name and path save the extension) if it exists
• Motivated by a need for better user interface in IDE when authoring FFI

[nwolverson]

Thanks for the PR, this looks good at a glance. Will look properly later.

[nwolverson]

I found some other issues with this, and merged with some fixes

• Document store as above
• Exception when handlers fire for JS files will spam the output pane, so trying to avoid that.

[aranchelk]

Thank you for making the changes, I've had that on my to-do list for weeks, apologies for not getting to it.