Tree-sitter grammar: support Function Type Reference

[OceanOak]

Changelog:

Tree-sitter-darklang
- Update the grammar to support function type reference

#5321

[StachuDotNet]

This might fit more naturally like this:

Suggested change

	fn_type_reference: $ =>
	prec.right(
	seq(
	$.type_reference,
	repeat1(
	prec.left(
	seq(
	field("symbol_arrow", alias("->", $.symbol)),
	$.type_reference,
	),
	),
	),
	),
	),
	fn_type_reference: $ =>
	prec.right(
	seq(
	repeat1(
	prec.left(
	seq(
	$.type_reference,
	field("symbol_arrow", alias("->", $.symbol)),
	),
	),
	),
	$.type_reference,
	),
	),

Feel free to disregard or leave for another PR, though

[StachuDotNet]

I'll merge as-is; if you'd like to refactor to the suggested grammar change, OK to do so in a follow-up PR. Maybe just adding a cleanup note would be good :)