Implement anchors for jumping to code

[xfbs]

Something like:

• https://diff.rs/view/serde/1.0.100/files/src/main.rs#L66
• https://diff.rs/view/serde/1.0.100/files/src/main.rs#fn:main

Allows for creating permalink to functions/lines for viewing crates.

[xfbs]

I pushed a commit that implements the first step in making this possible.

Still need to do more things:

• Make this work with single column view
• Automatically expand if anchor line is hidden
• Implement smooth scrolling (CSS scroll-behaviour: smooth)
• Implement scroll margin (CSS scroll-margin-top: 100px)
• Highlight anchor line
• Make line numbers clickable (link to anchor)