Extensions: known functionality bugs

[jcouv]

This issue is referenced in source.

• Disallow in nameof
• Fix anonymous type behavior (bug in test)
• Allow extern
• missing ERR_AmbiguousPrimaryConstructorParameterAsColorColorReceiver
• unreachable exception hit by vector extension properties PR (Roslyn hits "unreachable" exception when processing type forward to a class containing an extension group #79894)
• assertion in WasPropertyBackingFieldAccessChecked in ResolveAll_Query_Cast (not specific to new extensions, moved to Assert in WasPropertyBackingFieldAccessChecked #80008)
• improper tracking of used assemblies in function type scenario (LangVer_06, LangVer_12) (not specific to new extensions, only affects C# 12 or lower, won't fix)
• Removing this from scope (also affects region analysis public API)
• explicit entry point type using the skeleton metadata name should work (Validation_EntryPoint_03)

Operators:

• Address some known functionality gaps for extension operators #79167 - Allow 'extern' modifier
• Address another set of functionality gaps for extension operators #79227 - Collection of extension blocks depends on GetTypeMembersUnordered for namespace, which conditionally de-orders types for DEBUG only. There is an ordering difference for ambiguous operators in an error (DEBUG vs. RELEASE). A couple of tests hit this: Increment_056_Consumption_Checked and CompoundAssignment_064_Consumption_Checked.
• Address another set of functionality gaps for extension operators #79227 - ErrorCode.ERR_VoidError probably should not be reported for extension operators

Relates to test plan #76130