Parser: Ensure function and tag hook callbacks are callable upon registration

Parser: Ensure function and tag hook callbacks are callable upon registration

This is to safeguard against issues like T167238, where the callback was invalid,
but was only noticed when the magic word was used in an article.

In general, type checks should be applied when things get
registered for later use. Fail early, stay sane.

Change-Id: Ifb7005ee214829c98cec534261c0db7d13f50f35