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 {{{saysay}}} foo now renders as plain text instead of crashing
• invalid (invalid) {{{saysay}}} foo now renders as plain text instead of crashing
• cowsay (invalid) {{{saysay}}} foo will still crash as before

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25415