Add flow coverage support

[thymikee]

Summary

Flow coverage support.

StatusBar (clicking toggles uncovered code mode):

Uncovered code mode:

[orta]

Yeah, great - this looks good to me 👍

Very cool

[jacobk]

Is it possible to disable this feature?

I've experienced a big degradation in performance of vscode latley and I've tried to disable most other plugins but cannot find the root cause.

This seems like it could impact performance and I've had performance issues when trying to use flow coverage in the past too.

@thymikee Have you noticed anything similar?

[thymikee]

Coverage report is run for every diagnostic change, and by default the extension sets:

"flow.runOnEdit": true

which tells Flow to get diagnostics (coverage included) on every keystroke.
You can set it to false and see if that makes the difference.

Maybe we could defer it somehow, I'll see what can be done if I have some time to work on it.

[moberemk]

I'm also running into some weirdness with this where I'm getting "uncovered code" on files that don't even Javascript, like bash scripts.

[thymikee]

@jacobk @moberemk I've addressed couple of perf issues in #126

[moberemk]

My issue isn't a perf issue though, my issue is that the coverage check is being applied to the wrong types of files...

[thymikee]

That's fixed too.