Surface compiler warnings/errors in the UI

[Rich-Harris]

Errors will be shown in the error overlay, but it's not the friendliest thing in the world (hard to know where the error is without sourcemaps/code frame). Warnings won't be shown at all.

Realistically this means doing analysis in a separate worker, which will be finicky

[dummdidumm]

Would be solved as part of #26 if we get language tools to work in the browser - except you want a more prominent display similar to what the current REPL does.

[Rich-Harris]

It occurs to me that since compiler errors are being surfaced via the Vite overlay, warnings probably should be too. @dominikg do you know if that's possible?

[dominikg]

Possible from v-p-s via custom message on vite websocket. Either from onwarn or directly in compile util.

Needs a new config option (disabled by default) and possibly some ui/console.log implementation so it makes sense outside of learn.svelte.dev

[dominikg]

We already have some treatment for onwarn so adding it isn't a big deal. sveltejs/vite-plugin-svelte#372

[dominikg]

From a ux perspective it doesn't make sense to use a blocking overlay for warnings.

Not sure how to get these displayed directly in the editor pane but as long as the sourcemaps are ok it should work. Unfortunately preprocessor sourcemaps are hit and miss, so ymmv.

[Rich-Harris]

Implemented inline warnings. Errors are still a TODO, but warnings were the priority since these don't surface in the Vite overlay, and the tutorial text refers to a11y warnings very early on. This now appears on hover:

[dominikg]

ohhh, this is nice!

Idea: would it be possible to have a similar style for the warning on left and right pane so that the user immediately recognizes it?
The left (orange border, dark bg) is a bit more visually appealing to me but the borderless light orange bg might work better for the linine case on the right.