SCons: Add an option to enable the experimental ninja build backend

With this option turned on, if properly set up, SCons generates a
`build.ninja` file and quits. To actually build the engine, the user can
then call `ninja` with whatever options they might prefer (not
everything is yet transferred properly to this new generated file).

Ideally, the scons file should never be called again, as ninja
automatically detects any SCons build script change and invokes
the required commands to regenerate itself.

This approach speeds up incremental builds considerably, as it limits
SCons to code generation and uses ninja's extremely fast timestamp-based
file change detector.

.gitignore

@@ -35,6 +35,10 @@ bin
 compile_commands.json
 platform/windows/godot_res.res
 
+# Ninja build files
+build.ninja
+.ninja
+
 # Generated by Godot binary
 .import/
 /gdextension_interface.h