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/loader/README.md

@@ -0,0 +1,6 @@
+# `tree-sitter-loader`
+
+The `tree-sitter` command-line program will dynamically find and build grammars
+at runtime, if you have cloned the grammars' repositories to your local
+filesystem.  This helper crate implements that logic, so that you can use it in
+your own program analysis tools, as well.