Fix bug in MatchMapping class

[zrr1999]

This pull request fixes a bug in the MatchMapping class where a trailing comma was allowed without the presence of the **rest parameter. The bug was causing an error in the validation process. This PR ensures that a trailing comma is only allowed when the **rest parameter is present.

Reproduction:

import libcst as cst


cst.parse_module(r"""
match x:
    case {
        "a": a,
        **rest,
    }:
        print(a)
""")

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[zrr1999]

CLA signed

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.09%. Comparing base (f8a9b80) to head (2377202).
Report is 88 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1058   +/-   ##
=======================================
  Coverage   91.09%   91.09%           
=======================================
  Files         255      255           
  Lines       26564    26564           
=======================================
  Hits        24198    24198           
  Misses       2366     2366           

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