Identify rust-toolchain files as rust-toolchain

[jalaziz]

Introduce dedicated types for rust-toolchain and rust-toolchain.toml files. Cargo linting rules can change with the rust version, so it's helpful to match against the rust-toolchain files as a trigger for Rust linting hooks.

[asottile]

same feedback as your other PR: #460 (comment)

[jalaziz]

same feedback as your other PR: #460 (comment)

Yes, filtering by files can accomplishes the same things, but it's impossible to combine files and types with OR.

The docs encourage using types with:

Filtering with types provides several advantages over traditional filtering with files.

• no error-prone regular expressions
• files can be matched by their shebang (even when extensionless)
• symlinks / submodules can be easily ignored

Which encourages the use of types like rust, cargo, etc.

However, due to:

types, types_or, and files are evaluated together with AND when filtering. Tags within types are also evaluated using AND.

You can't mix and match types_or and files without including the for matching types in files.

Since Rust linting rules change with new versions of cargo (usually released alongside new versions of Rust), this is needed to avoid having to completely devolve into using files instead of types.