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/tests/helpers/fixtures.rs

@@ -1,9 +1,9 @@
-use crate::loader::Loader;
 use lazy_static::lazy_static;
 use std::fs;
 use std::path::{Path, PathBuf};
 use tree_sitter::Language;
 use tree_sitter_highlight::HighlightConfiguration;
+use tree_sitter_loader::Loader;
 
 include!("./dirs.rs");