[wip] allow extended syntaxed to reference updated variables

[keith-hall]

Hi,

Thanks very much for the PR implementing extends. Sorry to let it linger for so long.
I had some ideas of how we could get the match patterns from the base syntax definition (which has already been parsed) to use the variables from the derived syntax definition. And that is to store the original "raw regex" on the MatchPattern so we can reevaluate it with the "latest" variables. It is probably not the best way to do it, but I feel like at this stage, it is better to get something implemented, merge it and improve it later...
To this end, I added a test which should succeed when everything works as expected. I confess that I haven't actually had time to complete the implementation yet, but thought I'd push it here in case you or someone else wants to take over, or in case it pushes me to find more time for it myself...

[jalil-salame]

I've forgotten how this code works, but I'm happy to dive back into this. I'll get back to you within a week with a review as I'm a bit busy rn and need to re-read the code.

[jalil-salame]

This looks good, I still need to go back to the original PR and rebase it/update it. Lazily evaluating stuff seems like a better approach, definitely. The only problem with extends is that it needs access to the syntax file database, and I don't know if you have that information when highlighting.