Extensions: follow-up parsing issues

[jcouv]

This issue is referenced in source.

The list includes:

• we should probably have better error recovery in older LangVer so that we can offer a dedicated diagnostic to trigger UpgradeProject
• enforce that a ParameterSyntax cannot be created with both Type and Identifier missing

Would likely punt on the following:

• (string, string).M();
• (int a, int b).M();
• extension(scoped RS) { ... } (scoped is currently parsed as a type here; should still error though)

Relates to test plan #76130