Add Cargo.toml support

[kurtbuilds]

Created this to start progress on supporting better autocompletion for Cargo.toml files. Specifically:

• Autocomplete packages. My goal is to autocomplete based on a static list, the local package cache, and the crates api, falling back to only (2) when offline.
• Autocomplete package features
• Autocomplete package version
• Provide code actions like "Expand dependency specification"

All cargo.toml functionality is gated behind a cargo_toml feature.

By special casing cargo support, we solve existing bugs, most notably zed-industries/zed#15261 .

Credit to @frederik-uni and @washanhanzi for prior art in this space. Having this merged into taplo will improve the new user experience for users (more discoverable than installing 2 toml LSPs), so I think it makes sense to focus all efforts into taplo.