Fix mismatch between String and StringName in dictionary keys

[vnen]

Fixes the problems mixing String and StringName types when used as dictionary keys. This means users will have to be more careful of what they use as keys.

For reference, this is the expected behavior:

var dict = {"a": 1, &"b": 2}
print(dict["a"]) # should work
print(dict[&"a"]) # should NOT work
print(dict["b"]) # should NOT work
print(dict[&"b"]) # should work
print(dict.a) # should NOT work
print(dict.b) # should work

Note that shorthand notation (dict.a) assumes the key is a StringName). Also the new StringName literal notation: &"name".

This PR also makes sure the Lua-style dictionary literal uses StringName as keys, so the shorthand works too:

var dict = {a = 1, b = 2}
print(dict["a"]) # should NOt work
print(dict[&"a"]) # should work
print(dict["b"]) # should NOT work
print(dict[&"b"]) # should work
print(dict.a) # should work
print(dict.b) # should work

Fixes #44241
Supersedes #47899

[akien-mga]

Also supersedes #44310?

[vnen]

Also supersedes #44310?

That one does a different thing, which allows String and StringName to be the same. I believe we want to keep the idea that different types are always different, so that one probably shouldn't be merged, but it can be evaluated in its own merit (though I don't think it's the proper fix for this).

[akien-mga]

Thanks!

[AndreaCatania]

Thanks!!