Allow scope mappings to be refined via match rules

[queerviolet]

In the TextMate grammars, some tokens are highlighted differently based on their content. For example, document and exports and other common identifiers are colored differently than normal variables (even though they are technically normal variables).

Tree Sitter's highlighting system didn't support this... until now.

This PR adds tooling to TreeSitterLanguageMode enabling match rules on the RHS of a scope mapping in the atom grammar definition:

Example:

    'scopes':
         'identifier': [
            {exact: 'exports', scopes: 'global'},
            {match: '^[A-Z_]+$', scopes: 'constant'},
            {match: '^[A-Z]', scopes: 'constructor'},
            'variable'
          ],

The first rule that matches is applied, so SOME_CONSTANT will resolve to the scope constant even though it also matches /^[A-Z]/. String rules (such as variable above) always match, applying the specified scope (a nicety for default cases).