Add sort methods to PoolStringArray

[me2beats]

Describe the project you are working on:
gdscript plugins

Describe the problem or limitation you are having in your project:
There is sort() in Array, can't find it in PoolStringArray

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
sort() method in PoolStringArray could sort the array alphabetically

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:

PoolStringArray(["b","a"]).sort()
-> a b

If this enhancement will not be used often, can it be worked around with a few lines of script?:
I think this is possible, but I do not find it logical since there's sort() methods in Arrays

Is there a reason why this should be core and not an add-on in the asset library?:
should be a core feature

[Calinou]

This was added to the master branch in godotengine/godot#32144 already. It might be possible to remake that pull request for the 3.2 branch too.