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/javascript/detect.py

@@ -81,6 +81,7 @@ def configure(env):
             env.Append(LINKFLAGS=["--profiling-funcs"])
     else:  # "debug"
         env.Append(CPPDEFINES=["DEBUG_ENABLED"])
+        env.Append(CPPDEFINES=["DEV_ENABLED"])
         env.Append(CCFLAGS=["-O1", "-g"])
         env.Append(LINKFLAGS=["-O1", "-g"])
         env["use_assertions"] = True