Clickable error locations

[lydell]

Via the UI source maps feature we already have a way to open files from the browser. This re-uses that feature to make error locations in compile error messages clickable.

I needed to adjust the open editor feature a little to be able to re-use it. I ended up cleaning up that feature quite a bit, making it more prepared for vanilla Elm app use-cases (where we shouldn’t assume that the only source directory is called src for example). Bonus improvements:

• Now uses :: instead of List.append for performance. (Remember that we do this on every element attribute list.)
• Now uses a property instead of an attribute, so we don’t clutter the browser devtools web inspector view.

Note: When building, you need to add something like the following to StaticFiles.hs to bust cache:

x :: ()
x = ()