fix: support variadic functions in .pxd declarations

Modified _typedargslist rule to properly handle C-style variadic functions
using `...` syntax. This allows declaring functions with variable arguments
in various contexts:

- Standard library functions (printf, scanf)
- Custom variadic functions with/without regular parameters
- Variadic methods in cdef classes
- Function pointer typedefs with varargs support

Examples of now-supported syntax:
- cdef int printf(const char *format, ...)
- cdef void func(...)
- cdef int log(char *prefix, int count, ...)
- ctypedef int (*printf_t)(const char*, ...)

Added comprehensive test corpus covering various variadic function
declaration patterns.

grammar.js

@@ -508,6 +508,7 @@ module.exports = grammar(Python, {
             optional(seq(":", $.c_type)),
             optional(seq("=", choice($.expression, "*"))),
           )),
+          optional(seq(",", "...")),
           optional(","),
         ),
       ),