Add ast.ExternExportModifier { none, extern_, export_ } beside
CallingConvention; FnDecl.extern_export and VarDecl.is_extern/extern_name
fields (all defaulting to absent); and Parser.parseOptionalExternExport()
mirroring parseOptionalCallConv.
None of this is consumed by a decl path yet — no user-facing behavior
change, corpus diff empty. Two inline parser unit tests pin the helper's
keyword mapping and the field defaults. Phase 1.0 wires the helper into
the fn-decl path. lock commit.
62a3b46f6e