Allow compiling out the navigation module

[DarkKilauea]

Fixes: #36091

Alternative to #69881

This is a stripped down version of #69881 which addresses the core problem of not being able to compile Godot without the navigation module, without the complexity of supporting GDExtension nor introducing a project setting for changing the server implementation.

It leaves the existing system for registering navigation servers alone and mainly updates main.cpp to check if no server has been registered and if so, swapping it out for a dummy implementation instead of throwing an error. Since there is always a dummy implementation compiled, the linker will always find an implementation of NavigationServer3D and won't fail to link if the module is missing.

There are a few housekeeping items also in this PR, mostly swaps to equivalent types for readability, additional comments, missing override keywords, and a missing virtual keyword.

All changes are backwards compatible, with no changes to ClassDB nor any changes to the C++ API shape. We'd like to include this change in 4.0 if possible.

[Calinou]

We'd like to include this change in 4.0 if possible.

4.0 is in RC stage, so we can't afford making potentially risky changes now. That said, if this is considered a bug fix, this can be merged for 4.1 and cherry-picked to a 4.0.x patch release.

[DarkKilauea]

@Calinou That's fine, just keep in mind that anyone trying to compile out the navigation module will run into issues until a fix is merged. This effectively makes the navigation module mandatory.

[akien-mga]

Seems fine.

[akien-mga]

Thanks!