Use EditorPlugin.build to compile Dart .dills

[fuzzybinary]

Instead of loading Dart from source during game launch, it might be faster to compile our Dart code into a Kernel and load that if it's available.

To do this, we would modify GodotDartEditorPlugin to override _build, and call into the Dart executable to compile the kernel.

Alternately, Dart_CompileToKernel may work as well, and wouldn't require we ask the shell to perform the work.

This should really only be done if it is faster to do this when a game over waiting for the script compile when the game launches.