Add syntax highlighting for TOML files.

[ab22593k]

Because rust is officially support Tom's Obvious, Minimal Language (TOML), and usually we can not find a project without a Cargo.toml/lock files. I suggest to add a syntax highlighting without needs for external extensions.

[bjorn3]

It would also be nice to have completions for Cargo.toml.

[SomeoneToIgnore]

Also formattting and intelligent hints and actions: outdated imports detection, incorrectl (missing) package name and versions detection, etc.

[matklad]

I did some experiments with this in https://github.com/matklad/tom. It should be moderately straight-forward to update that to modern rowan, but it requires work.

[tamasfe]

I've made a library that does this. I originally started it for Rust Analyzer, but it ended up being a bit too messy, and needs some internal design changes and refactors right now. However it mostly works, and there's a VSCode extension based on it.

Even if not the library itself, you might still find parts of it useful. You could probably also add the extension as a dependency until this gets resolved.

[lnicola]

Besides syntax, an assist to convert from foo = "1.0" to foo = { version = "1.0", default-features = false, features = ["<|>"] would be really nice.