More icons

[Kixiron]

Depends on #790

Source pages have a very small subset of icons and I want to extend them with more, e.g. markdown icons for markdown files and lock icons for Cargo.locks

[Nemo157]

Related request, change the PR count icon from code-branch to code-merge, I keep thinking that it's the count of forks or something instead.

(I didn't check if code-merge is available on FA4, which is why I'm putting this here)