Allow Trailing Comma In match_expression_list

[Vehmloewff]

... It's valid PHP, apparently.

php -r 'echo match ("Hello") {
        "Hi", "Hello", => true,
        "Bye", => false,
};'

Checklist:

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

[cfroystad]

Thanks for fixing this and also adding a test 🙂

This is a reminder that we should check the implementation and not just the RFC when implementing support for new features here (I'm a fault here). This is not mentioned in the RFC, but is part of the original implementation. Thanks for the fix!

We'll land this after #192 is landed since that moves stuff around a bit

[calebdw]

rebased locally and merged to master