[Version 14.0] Feature support for extension members

[RexJaeschke]

This is Rex's adaptation of the corresponding MS proposal #1 and MS proposal #2.

1. Extension indexer support will be added in V15.
2. Although the MS spec uses the term “receiver parameter” and related grammar rule receiver_parameter, I renamed that as “extension receiver parameter” and extension_ receiver_parameter, respectively. Here’s why:

• All the other extension-related grammar rule names—extension_declaration, extension_body, and extension_member_declaration—have the prefix “extension”.
• We also use the terms extension block and extension member.
• Related compiler error messages typically use the prefix “extension”.
• “receiver” is already used in unrelated contexts in the spec.

1. The following sections in the MS proposal appear to be about implementation detail. As such, nothing from them was include in this PR: Lowering and Metadata for declarations.
2. I did not add anything about the implementation machinery for making the expansion block a nested type.
3. The extension operators subclause mentions the type MethodInfo. See this V11 feature PR comment re this type.