Handle `extern type` declarations

[CohenArthur]

• Cannot parse extern type declarations #1917
• Add AST node for ExternalType #1923
• Add HIR node for ExternalType
• Add feature gating behind #![feature(extern_types)] #1925
• Handle typechecking properly
  • refer to RFC, extern types have a lot of subtleties
• Compile extern types properly
• Allow visibility modifiers for extern types #2897