Parse empty type params

The two examples below fail to parse.

I think we should loosen the parsing requirements to allow empty type parameters, and then let the library user (Steep, Sorbet) decide how to handle that.

These cases are reachable if you're editing a file and want to rename a type parameter name. You would delete what's already there (leaving the [] empty), before you start typing the new name.

If we choose to leave this as an error in the RBS parser, it would be nice to at least have a more human-readable error messages.

Examples from Sorbet

Empty type parameters on a type (Sorbet.run):

#: []
#  ^^ ❌ expected a token `pLBRACKET`
class NotActuallyGeneric
end

https://github.com/ruby/rbs/blob/e1373f0182fdfdc30d78be9b591ac188b8af7603/src/parser.c#L3341-L3345

Empty type parameters on a method (Sorbet.run):

#: [] (T) -> T
#  ^^ ❌ expected a token `pARROW`
def identity(x) = x

Minimal repro

require "rbs"

RBS::Parser.parse_type_params "[]"
# /.../gems/rbs-4.0.2/lib/rbs/parser_aux.rb:43:in 'RBS::Parser._parse_type_params': a.rbs:1:0...1:2: # Syntax error: expected a token `pLBRACKET`, token=`[]` (pAREF_OPR) (RBS::ParsingError)
#
#  []
#  ^^

RBS::Parser.parse_method_type("[] () -> void")
# /.../gems/rbs-4.0.2/lib/rbs/parser_aux.rb:17:in 'RBS::Parser._parse_method_type': a.rbs:1:0...1:2: Syntax error: expected a token `pARROW`, token=`[]` (pAREF_OPR) (RBS::ParsingError)
#
#  [] () -> void
#  ^

Start with src/parser.c at the linked type-parameter parsing code, then inspect lib/rbs/parser_aux.rb and the parse_type_params and parse_method_type entry points from the repro. Confirm whether [] should parse for both examples or only receive clearer errors, and verify that the chosen behavior is covered by the parser tests.