rust: Extract runtime language detection into separate crate

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!

cli/src/lib.rs

@@ -1,7 +1,6 @@
 pub mod config;
 pub mod generate;
 pub mod highlight;
-pub mod loader;
 pub mod logger;
 pub mod parse;
 pub mod query;