Support `toml` marshall/unmarshall

[aignas]

For the same reasons that #219. Python and Rust use toml as the format to store metadata and having native implementation to encode and decode such files could potentially lead to easier implementation of bzlmod extensions and/or repository rules, where we would need to read a Cargo.toml or pyproject.toml or a poetry or pdm lockfile to construct valid bazel targets.

[stepancheg]

This is the repo about starlark specification, not one of several starlark implementations.

I think TOML does not belong to the common starlark spec.