Rename server free functions to free_rid to match exposed API

[aaronfranke]

In the current master before this PR, the servers had these functions registered with the name "free_rid":

ClassDB::bind_method(D_METHOD("free_rid", "rid"), &NavigationServer2D::free);
ClassDB::bind_method(D_METHOD("free_rid", "rid"), &NavigationServer3D::free);
ClassDB::bind_method(D_METHOD("free_rid", "rid"), &PhysicsServer2D::free);
ClassDB::bind_method(D_METHOD("free_rid", "rid"), &PhysicsServer3D::free);
ClassDB::bind_method(D_METHOD("free_rid", "rid"), &RenderingDevice::free);
ClassDB::bind_method(D_METHOD("free_rid", "rid"), &RenderingServer::free);
ClassDB::bind_method(D_METHOD("free_rid", "rid"), &TextServer::free_rid);

I was working on some C++ code that uses RenderingServer and trying to make it compatible with both in-engine module code and GDExtension C++ godot-cpp, and I found that the exposed API is different from the internal API for no reason. The name free_rid is clearer and avoids confusion with freeing Objects. Furthermore, TextServer already uses the name free_rid for its internal function.

This PR renames the internal free function on NavigationServer*D, PhysicsServer*D, RenderingDevice, and RenderingServer to free_rid to match the exposed API in GDScript and GDExtension and match TextServer.

Third-party modules can support multiple Godot versions by using free_rid and conditionally having #define free_rid free depending on the Godot version in files that use the modified servers.

[Ivorforce]

I fully agree, it's definitely worth the churn to make APIs consistent in- and externally.
Lately we've been a bit more mindful about API breakage. It would be possible to expose a [[deprecated]] void free function in addition the free_rid to make the transition moother for modules. What do you think?

[Repiteo]

Stylistic nitpick aside, this looks good!

[Repiteo]

Needs rebase

[Repiteo]

Needs another rebase 🙃

[aaronfranke]

@Repiteo Rebased and retested.

[Repiteo]

I bet you'll never guess what I'm gonna say

[Repiteo]

Thanks!