require using !!/watch-force to watch expressions with a lookahead at the start

[micsthepick]

Is your feature request related to a problem? Please describe.

When adding watched keywords, the usual advice from Makyen is to not start with a lookahead for performance reasons, https://chat.stackexchange.com/transcript/message/68148509 and the kerfuffle that follows is a prime example of how this can turn problematic fast.

Describe the solution you'd like

I propose making !!/watch force require you to use -force if you REALLY want to start a regex with (?! or (?= or variants, but am open to suggestions.

Describe alternatives you've considered

might want to make it a bit more robust?

Additional context

cc @makyen

[micsthepick]

I also happened on https://chat.stackexchange.com/transcript/11540?m=68054569 - and I think setting flags globally should be detected too?

[micsthepick]

https://chat.stackexchange.com/transcript/message/67655825 is also a consideration (character class quantified at start of regex)