VariantParser: Fix reading negated identifiers, replace inf_neg with -inf

[aaronfranke]

This PR is a subset of PR #100414 that fixes VariantParser to allow for reading negative identifiers in .tscn files, and also changes the serialization of negative infinity from inf_neg to just -inf. In the current master, the code assumes that anything starting with - is a finite number.

To test this PR, I made a new project with a Node3D and gave it a tool script like this:

@tool
extends Node3D

func _ready():
	print(position) # Prints (0.0, 0.0, 0.0) the first time, then (inf, -inf, nan) after the scene is saved and re-opened.
	position.x = INF
	position.y = -INF
	position.z = NAN

Note: The reason I previously did not have this change separated out is because I wanted to go straight from inf_neg to -INF instead of going through -inf first, but it seems PR #100414 was a bit too much to review in one go.

Note: We should also cherry-pick most of these changes to older Godot versions, except for the inf_neg writing change, so that older versions can read new files saved with -inf, but the old versions should keep writing inf_neg to match older patch releases in those minor versions. However, this is not vital since -inf is a very rare value to serialize to a .tscn file, especially because it is currently not possible to read or write such values in the editor inspector.

[Repiteo]

Makes sense to me!

[Repiteo]

Thanks!

[nikitalita]

the rtos_fix change ends up breaking compatibility with version 3 resources, when we explicitly support that in VariantWriter::write when the p_compat flag is true; we ought to respect that and write inf_neg if it is true.