Add anchor for intra-doc links to associated items

[ncrothers]

Fixes #8487, which points out a difference between how rustdoc and rust-analyzer rewrites intra-doc links in the markdown. rust-analyzer currently does not properly add the anchor to reference items associated with a type, such as struct fields or methods in an impl block.

This PR implements the fix to add the appropriate anchor for the associated item at the end of the URL, becoming aligned with how rustdoc handles it.

• rewrite_intra_doc_link
  • If no anchor is provided for the link, falls back on the fragment from filename_and_frag_for_def
• filename_and_frag_for_def
  • Modify Definition::Variant to follow the same procedure as Definition::Field, early returning with the fragment specified
• doc_links::tests
  • Add a test to validate rewriting for all types of associated items (fields, methods, trait methods, associated constants, associated types)

[Veykril]

Thanks!