Interface heritage: parse repeated extends clauses

tsc parses an interface with REPEATED extends clauses () — the parser accepts them, the checker reports the
duplicate. Mono's single opt('extends', sep(Type,',')) clause rejected the
second , so the construct only 'parsed' by splitting into garbage
statements. many('extends', sep(Type,',')) mirrors tsc and produces the
correct interface-with-heritage tree (parserInterfaceDeclaration1-4,
interfaceThatInheritsFromItself). Accept-neutral on the corpus (the split
path already accepted these), gates 34/34, corpus parity 401/401,
gate:treesitter 96.0%; also a prerequisite for statement-level ASI (Task
#24) which otherwise rejects these.

Author: johnsoncodehk