fix: parse doc link with turbofish generics

[dimxy]

This PR refactors doc_modpath_from_str fn marked with "FIXME" and fixes parsing of links with turbofish.

There are a few issues with the existing function:

• it is implemented as ad-hoc code although the syntax crate contains ready-to-use features to parse type paths.
• adding tuple index to mod path (which is invalid there) looks odd.
• there is a bug: paths with turbofish generics are parsed incorrectly. For example, "S::<i32>::f" is parsed as
  ModPath { kind: Plain, segments: [Name { symbol: "S", ctx: () }, Name { symbol: "", ctx: () }, Name { symbol: "f", ctx: () }] } (extra empty "symbol" in the middle) and cannot be navigated in r-a GUI.

The proposed solution:

• adds impl ast::Type { fn parse(..) } similar to existing ast::Expr.
• implements a new mod_path_from_doc_link fn with uses new ast::Type::parse and existing ModPath::from_src.
• handles tuple file index separately from the modpath, in the caller function.
• correctly parses turbofish generics (now trackable in r-a GUI).

Disclaimer: Claude Code was used to help with code analysis, reviewing changes and suggesting tests.