feat: use cargo.toml metadata for build paramaters

[schell]

This adds a toml subcommand that parses a Cargo.toml file at a given path. Either a workspace toml or a package toml are accepted. The toml is parsed to retrieve parameters used in a cargo gpu build invocation.

Fixes #6.

Depends on

• fix: provide spirv-builder with a path to a specific target-spec file #13