GDScript: Partially allow member lookup on invalid scripts

[rune-scape]

this was supposed to be my fix for this regression: #92534
it does fix it separately from #92544

rethinking the changes i made in #92035 and #90601

• always default initializes static variables right after compiling a class, safer and less restrictive
• doesn't invalidate script when dependent scripts don't compile/resolve, as this may have caused other (non segfault) problems i didn't imagine
• each GDScriptFunction on invalid scripts are protected with a check to see if the script is valid instead of returning too early

this all means that invalid scripts can still have their members safely queried and possibly even native base functions called if there are instances with the invalid script attached
and that a script is not 'invalid' if only a depended script is 'invalid' (as it was before)

fixes #92610

[CsloudX]

Thanks, i'm tested with this pull, it fixed #92610

[dalexeev]

I'm not sure how critical this is, but there may be a situation where the current script is not yet valid, but the base script is already valid. In this case, static function lookup will work inconsistently for "inherited" (actually shadowed) static methods. Until the current script compiles successfully, the lookup will find the base method, and then the current class method.

[bitwes]

I tested this PR with GUT and it fixed the issue.

Minor note
In an overly complicated scenario (which I can describe if necessary), I had to add an additional check of is_instance_valid where I was previously just checking for null. Without this check I get Attempt to call function 'get_current_test_object' in base 'previously freed' on a null instance.

For all I know, this could be an issue with GUT that was fixed in Godot, and not a Godot issue. It's a non issue for GUT, I am just documenting my experience with this PR.

[AThousandShips]

Makes sense and looks good in theory, but TIWAGOS

[akien-mga]

As discussed in the release management meeting yesterday, let's go ahead with that change even though GDScript contributors are not available right now for an in-depth review.

This is a follow up to rune's own changes and was tested by several affected users, so it should be good to go and test more widely in the next beta.

[akien-mga]

Thanks!