GDType: Consolidate integer constants in property map

[DeeJayLSP]

• Follow up of Move property maps from ClassDB to GDType. Accelerate Object property access 1.6x #122596

What problem(s) does this PR solve?

#112596 introduced property maps in GDType, which made integer constants exist both in a dedicated integer constant map and the property map.

Like how it's done with setgets, it should be possible to get the property map, validate that the type is integer constant, then get the integer from its payload.

Memory usage test

• Build command: scons target=editor linker=mold dev_build=yes optimize=debug debug_symbols=yes werror=yes tests=yes

Test script:

extends Node

func _ready():
	print(OS.get_static_memory_usage())
	await get_tree().process_frame
	get_tree().quit()

On a simple project that just prints OS.get_static_memory_usage() then quits, I got the following from running in headless mode:

Build	Memory usage (bytes)
master	88894464
PR	86315132

Effectively a reduction of 2.46 MiB.

Additional information

The aforementioned PR mentions the new maps added over 20MB in RAM usage. This is just my attempt to mitigate part of it, but it could be superseded as soon as someone comes with a better/bigger solution to the problem.