Synchronize NavigationRegion2D's modulate to its debug elements.

[Daylily-Zeleen]

The main purpose is to adjust the display of NavigationRegion2D's debug mesh, to distinguish or reduce its visibility, and to avoid it being cluttered in the editor.

[smix8]

Hi, thanks for working on this. Unfortunate timing as for now any navigation debug new feature or major change is set on hold until godotengine/godot-proposals#7208 is finished.

The navigation debug is in the transition to be server handled. This is already nothing but cumbersome to implement with all the node and scenetree (and especially Node2D) quirks that have been stacked up to the stratosphere over the years so we are not adding new debug features for now to not make things even more complex.

I am ok with adding customization in the spirit of this PR at some point for region navmeshes, e.g. like what navigation agent has for its paths but it will likely require a very different implementation with far more server API and internals involved. It is too early to plan for this but we could keep this PR open and discuss it at a later point when that info becomes available.

For navigation we insist on feature parity between 2D and 3D where feasible so such a customization would require additonal work on the 3D end as well. The 3D part does not need to be necessary be made by the same person or same PR.

[Daylily-Zeleen]

@smix8 the proposal that your mention about looks great, but it is open over one year.
If it can't be implemented before 4.4 stable, I think this pr can be merge first.
In my personal experience, this issue is very frustrating when setting up large and complex scenes. I think it should be solved as soon as possible.
On the other hand, this pr does not changes too much code, I think it can be well coordinated with your proposal in future.
If this pr is unable to be compatible in the future, we can just remove it because NavegationRegion2D has a mark of experimental, some changes are acceptable.