Spawn external from within a rule

This idea came from #2095

Solar: It makes me thinking what command character we'd (eventually) use for executing an external mode function from a ruleset, though - 'e' could have stood for "external" - but perhaps this should in fact be a separate topic

As for external being called as a rule, I really only see calling a filter function, to modify the word or reject it.

As for the rule to use, these are already spoken for (where there are letters available of either case, there is a gap)

A cCdDeE f iI kKlL M oOpPqQ rRsStTuUvV xXyYzZ { } [ ] () <> _ ' : / $ ^ # % @ ! = \ * + - . , 12 4 6

So we can see we are getting pretty 'tight' on letters. For calling an external function, could we view this as a 'spawn', and wrap it in sh syntax? function ?

The ` is not used, and seems a 'safe' bet. That way, we could even comment it could be a extern function or even a system spawn (Yes, I know solar will not agree, but we can wish, lol).

So a rule such as:

\]\]$[a-f0-9]$[ -~]ccorp_complexity``
would chop last 2 letters, append a hex, then some other ascii char, then case the word, and then finally call the corp_complexity 'script/external' which would accept or cause the final word to be dropped as not meeting some corp standard.

The issue names no implementation files, tests, or entry points. Start by locating the ruleset parser and existing external or filter-function handling, then clarify the proposed syntax and execution behavior; done would require an agreed design and corresponding implementation and tests.