Highlight all exit points (such as `return`/`break`/`continue`) in a function when the text cursor is on one

[DanTup]

Slightly related to #50532. When the text cursor is in a return keyword, other returns for the same function should also be highlighted (via DocumentHighlights for LSP).

I think it also makes sense to do the same for yield, break and continue.

Question: Should we highlight both break and continues that have the same target? (I think yes.. @bwilkerson?)

[bwilkerson]

If we're going to highlight returns, and I hope we do, then highlighting yields makes sense.

I suspect that there's less utility in highlighting break and continue, but it would be consistent, so I'm fine with doing that.