GDScript: Improve type inference for arrays and dictionaries

[dalexeev]

See #83660 (comment).

Infer hard Variant for elements of untyped arrays and dictionary keys/values if they are of hard type.

TODO: Subscript resolution:

var hard_dict := {key = "value"}
var hard_array := ["elem"]

var a := hard_dict["key"] # INFERENCE_ON_VARIANT warning, not error.
var b := hard_dict.key # INFERENCE_ON_VARIANT warning, not error.
var c := hard_array[0] # INFERENCE_ON_VARIANT warning, not error.