LibGodot: Core - Build Godot Engine as a Library

* Add a new GodotInstance GDCLASS that provides startup and iteration commands to control a Godot instance.
* Adds a libgodot_create_godot_instance entry point that creates a new Godot instance and returns a GodotInstance object.
* Adds a libgodot_destroy_godot_instance entry point that destroys the Godot instance.

Sample Apps: https://github.com/migeran/libgodot_project

Developed by [Migeran](https://migeran.com)

Sponsors & Acknowledgements:

* Initial development sponsored by [Smirk Software](https://www.smirk.gg/)
* Rebasing to Godot 4.3 and further development sponsored by [Xibbon Inc.](https://xibbon.com)
* The GDExtension registration of the host process & build system changes were based
  on @Faolan-Rad's LibGodot PR: https://github.com/godotengine/godot/pull/72883
* Thanks to Ben Rog-Wilhelm (Zorbathut) for creating a smaller, minimal version for easier review.
* Thanks to Ernest Lee (iFire) for his support

Co-Authored-By: Gabor Koncz <gabor.koncz@migeran.com>
Co-Authored-By: Ben Rog-Wilhelm <zorba-github@pavlovian.net>

SConstruct

@@ -264,6 +264,14 @@ opts.Add(
         True,
     )
 )
+opts.Add(
+    EnumVariable(
+        "library_type",
+        "Build library type",
+        "executable",
+        ("executable", "static_library", "shared_library"),
+    )
+)
 
 # Thirdparty libraries
 opts.Add(BoolVariable("builtin_brotli", "Use the built-in Brotli library", True))
@@ -556,6 +564,13 @@ if not env["deprecated"]:
 if env["precision"] == "double":
     env.Append(CPPDEFINES=["REAL_T_IS_DOUBLE"])
 
+# Library Support
+if env["library_type"] != "executable":
+    if "library" not in env.get("supported", []):
+        print_error(f"Library builds unsupported for {env['platform']}")
+        Exit(255)
+    env.Append(CPPDEFINES=["LIBGODOT_ENABLED"])
+
 # Default num_jobs to local cpu count if not user specified.
 # SCons has a peculiarity where user-specified options won't be overridden
 # by SetOption, so we can rely on this to know if we should use our default.