Add syntax error for empty type parameter list

[dhruvmanila]

Summary

(I'm pretty sure I added this in the parser re-write but must've got lost in the rebase?)

This PR raises a syntax error if the type parameter list is empty.

As per the grammar, there should be at least one type parameter:

type_params: 
    | invalid_type_params
    | '[' type_param_seq ']' 

type_param_seq: ','.type_param+ [','] 

Verified via the builtin ast module as well:

$ python3.13 -m ast parser/_.py
Traceback (most recent call last):
  [..]
  File "parser/_.py", line 1
    def foo[]():
            ^
SyntaxError: Type parameter list cannot be empty

Test Plan

Add inline test cases and update the snapshots.

[github-actions]

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