[3.x] Improve GDScript autocomplete type detection with the most-specific types available

[Macksaur]

See #39228 #53318 #61978. This change would fix those issues in 3.x. It may fix others that depend on autocomplete type information.

This change also improves #31818. The correct signals now show up in obj.connect(<autocomplete here>) in many new scenarios.

[Macksaur]

Test Case: #62074 improve-autocomplete-specific-types.zip

In Testing.tscn open the root node script and place your cursor at the end of each line in _ready() and autocomplete the variables. Observe they match the names in the comments. Also try to autocomplete the signal names in connect()!

Also you can run the test without this PR in 3.x/3.4! Note the stark difference in completions offered. There are a lot of missing completions and significantly reduced signals available to autocomplete in 3.x without this PR.

Here's a snippet of the completions offered to signals.

[Macksaur]

After too much time contemplating #58676 I felt that there was more opportunity to push this further. I refactored the changes into the GDScriptParser and started to take advantage of the increased availability of type traits.

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Instead of overly restrictive compiler errors, is/as operators are now much more relaxed (and behave like (thing as Object) as Derived) and instead only produce warnings if the type conversion is thought to be impossible.

Also, ternary if can now correctly discern the common type between its arguments and now provides only the correct completions that are guaranteed to be available. INCOMPATIBLE_TERNARY is still, of course, a valid warning when the types can't be coerced. (randf() is here used to prevent constant folding into a specific-type in the test).

These particular changes have been made in part to gather comments and allow for thinking out loud around the subject. They may be slightly too much for 3.x despite not introducing any breaking changes (🤔?). The original PR can always be cherry picked and cleaned up form 24bb5fd.

This all looks to be very simple to add to 4.0 if this is a desirable PR.