Add forwarded parameters/arguments

[aibaars]

Adds support for forwarded arguments/parameters (...), for example:

def foo(...)
end

def foo(a, b, ...)
end

Checklist:

• All tests pass in CI.
• There are sufficient tests for the new fix/feature.
• Grammar rules have not been renamed unless absolutely necessary.
• The conflicts section hasn't grown too much.
• The parser size hasn't grown too much (check the value of STATE_COUNT in src/parser.c).

[maxbrunsfeld]

Could you add something to this test so that forward_argument is used too?