Change GDScriptDataType container_element_type to vector container

[Repiteo]

Gonna start extracting bits from my typed dictionary PR (#78656) so the changes can be integrated more smoothly, starting with changing container_element_type from a pointer to a vector of pointers. This allows support for multiple types in the container syntax, without drastically overhauling the current implementations

While mostly superfluous in our current context of only having typed arrays, this will allow other type collections (eg: dictionaries) to be integrated straight away without needing to change the existing system. It also adds support for highlighting all container elements as types, and will recognize if a container is over/under capacity (only for arrays atm, will specify it requires exactly one type)

[adamscott]

Decision made at the GDScript Team Meeting that we want this PR to make it for 4.2, even if there's nothing front-facing to a user, will take time to review it this week. This PR will facilitate future development of typed dictionaries.

[vnen]

Looks good to me overall, just a couple of nitpicks.

[Repiteo]

Haven't seen/heard anything to the contrary, so the containers are now changed to be public & not use pointers. This certainly simplified the code, with several areas being made redundant in the process & cleaned up as a result

[vnen]

LGTM

[vnen]

Regarding the pointer, as the comment said it was for memory management. I didn't to construct and copy an empty container type every time, since it's more likely that it does not exist. Being a pointer it can be set only when needed, which is also why it was private.

With a vector this is less of a concern because the vector can be empty.

[adamscott]

Approved during the GDScript meeting, just have to apply the suggestions by @dalexeev. Thanks!

[adamscott]

Thanks, @Repiteo !

[YuriSizov]

Thanks!