R1:7868ab3754fa

Make InterpreterBlockRule regex only match on valid interpreter names

Summary:
With this patch, the underlying exception described in T15372#8537 still remains. However, with this patch, the bug is more contained as it is not triggered when not calling an interpreter (`cowsay`, `figlet`), so Phorge does not crash rendering `noValidInterpreter {{{foo}}} bar` lines but renders them as is (for whatever reasons such lines may exist).

See T15372

Test Plan:
Enter strings into a comment:
* `invalid…