Consider styling go-to-definition links using faint underline

[camelid]

Basically, we could style the links similarly to Haskell's equivalent to Rustdoc, Haddock. They use a faint underline, and then add background color when the link is hovered:

I like how it's very subtle in Haddock, so it doesn't affect the reading experience, but it's strong enough to think, "Hey, I bet I can click that link!"

IIUC, nearly all of the identifiers should be clickable in the generated source view, so we just want to give a hint that the links are clickable without distracting from the reading view.

Originally posted by @camelid in #88111 (comment)

[jsha]

Instead, what about using the same color conventions in source view that we do in doc view? As in, the last token of a path (Baz in foo::bar::Baz) is linkified and colored.

There's a slight conflict here:

• the source view is currently color-highlighted the way an IDE highlights: keywords are colorized; identifiers and types are unmarked.
• the doc view is color-highlighted in an inverse way: keywords are unmarked; identifiers and types are colorized.

I think it makes the most sense to follow the doc view conventions here, in part because colorizing the identifiers and types can act as the indicator that they are clickable. This would be consistent with the doc pages, and would avoid adding distracting emphasis on the source page.

If we adopt this, for consistency we'd want to also colorize and linkify identifiers when they are declared, like idx on the first line of this example:

[camelid]

Hmm, it's an interesting idea. FWIW, some IDEs do color identifiers (semantic highlighting).