This patch adds a new `tree-sitter-loader` crate, which holds the CLI's
logic for finding and building local grammar definitions at runtime.
This allows other command-line tools to use this logic too!
The .git file can contain a reference to the actual git directory as is commonly the case for a submodule[1]. When this is the case, read the .git file to discover the actual git directory.
* cli/build.rs: Read the .git file to discover the git directory
[1] https://git-scm.com/docs/gitrepository-layout
