SCons: Add DEV_ENABLED defines for target=debug builds

This will allow adding developer checks which will be fully compiled out in
user builds, unlike `DEBUG_ENABLED` which is included in debug tempates and
the editor builds.

This define is not used yet, but we'll soon add code that uses it, and change
some existing `DEBUG_ENABLED` checks to be performed only in dev builds.

Related to https://github.com/godotengine/godot-proposals/issues/3371.

platform/uwp/detect.py

@@ -71,6 +71,7 @@ def configure(env):
         env.Append(CCFLAGS=["/Zi"])
         env.Append(CCFLAGS=["/MDd"])
         env.Append(CPPDEFINES=["DEBUG_ENABLED"])
+        env.Append(CPPDEFINES=["DEV_ENABLED"])
         env.Append(LINKFLAGS=["/SUBSYSTEM:CONSOLE"])
         env.Append(LINKFLAGS=["/DEBUG"])