Array: Relax slice bound checks to properly handle negative indices

[akien-mga]

The same is done for Vector (and thus Packed*Array).

begin and end can now take any value and will be clamped to
[-size(), size()]. Negative values are a shorthand for indexing the array
from the last element upward.

end is given a default INT_MAX value (which will be clamped to size())
so that the end parameter can be omitted to go from begin to the max size
of the array.

This makes slice works similarly to numpy's and JavaScript's.

Fixes #56661.

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TODOs:

• Write docs for Packed*Arrays
• Update/add more unit tests for slice
• Review C# and GDNative counterparts, if any

[akien-mga]

@godotengine/mono I don't see any reference to Array.slice() in the Godot glue, is that expected?

[akien-mga]

This should be good to review. It works fine, the main question is whether we're fine with relaxing these checks and using INT_MAX as default value. I used INT_MAX instead of INT32_MAX or INT64_MAX as array/vector indices use int, but since GDScript int is explicitly int64_t there's a somewhat weird inconsistency here (but that goes through the whole codebase, aside from critical paths which have been improved recently like for FileAccess).