Add anchors to arguments

[hadley]

Fixes #2228

@glin I can't request a review, but I'd appreciate your thoughts on this PR, particularly the visual styling.

[hadley]

Sample styling:

[glin]

I didn't really review the code changes, but functionally this looks good to me. I've definitely wished it was possible to link to arguments a few times in the past.

For styling, I did think the border stood out a bit at first glance. Maybe a thinner border would look good, or maybe more spacing between the border and text?

Or alternatively, what I see often on doc sites is that the active anchor is kept visible and sometimes visually emphasized. This would probably be a more subtle option. Here's an example from some docs I was just looking at: https://docs.posit.co/rspm/news/package-manager/#posit-package-manager-2023120

No strong opinions on this though. But also as a side note, I like the visual indicator for active anchors, and think it would be useful to add to heading anchors at some point too.

[hadley]

I like the idea of making the anchor links stay visible in general; but I think arguments will need a bit more than. But I take your point that this visual treatment might be a bit heavy, and I'll brainstorm some alternatives.

[hadley]

I tried a few other visual treatments (including a light background and changing the text colour), but it's hard to be prominent enough without causing potential accessibility issues. So I refined the initial approach using a narrower border, more whitespace, and switching colours (to the primary colour, which is also used for links, giving a nicer visual connection IMO). I also liked the idea of showing the anchor, which lead to a few visual tweaks there too.

[glin]

The changes look great! The added whitespace and narrower border help a lot for me. Also, definitely appreciate the anchor change.