Meta feature tracking: Language server support for `Cargo.toml` file

This issue is an attempt to put all information regarding potential Cargo.toml support in one place.

Blockers

There is no TOML support in RA, and needs a format-preserving parser. toml-edit is maintained along with toml and probably the best choice.

Potential features

• Autocomplete or schema help (reference: https://doc.rust-lang.org/cargo/reference/manifest.html)
  • Autocomplete keys
  • Suggest valid values for fixed keys like edition, rust-version, license
  • Suggest in-workspace files for license-file, readme, build
  • Autocomplete dependencies (also handled by the crates extension)
  • Autocomplete dependency features
• Schema validation and warnings/errors, based on the above
• Quick actions & refactoring
  • "Move feature to dev-dependencies/dependencies/build-dependencies"
  • "Update crate to latest compatible version"
  • "Update crate to latest incompatible version"
  • Toggle no-default-features
  • Refactor simple dependencies to objects and back, like RustRover does https://github.com/rust-lang/rust-analyzer/issues/15741#issuecomment-1761881522
    
• Add crates to Cargo.toml from a use statement #10904
• Lower priority features that are already well-handled with other ecosystem extensions:
  • Link to crates like the crates extension: https://github.com/rust-lang/rust-analyzer/issues/13532
  • Add syntax highlighting to TOML files #3565. I think the syntax highlighting is not much of a concern since Even better TOML does it well, but there are some good general suggestions in the issue

See also the label https://github.com/rust-lang/rust-analyzer/labels/S-needs-toml

This is a meta-tracking issue rather than a self-contained task. Start by reading the TOML support blocker, the Cargo manifest reference, and the linked issues for crates integration, then choose a single unchecked feature with a defined scope. Done means that one Cargo.toml language-server capability is implemented and its behavior is verified.