They used to be called e.g. `ts_language_python`. Now that there
are APIs that deal with the `TSLanguage` objects themselves, such
as `ts_language_symbol_count`, the old names were a little confusing.
Tokens that are defined in the grammar's rules may now be included in the
externals list also, so that external scanners can check if they are valid
lookaheads or not, and if so, can return them to the parser if needed.
