Improve npm run watch with flags

[EvanHerman]

npm run watch will now only generate standard .css files. This means that, when developing with Go, you will need SCRIPT_DEBUG defined and set to true.
define( 'SCRIPT_DEBUG', true );

You can watch and generate .min.css, -rtl.css and -rtl.min.css files by adding flags to the npm run watch command.

Generate standard CSS assets

npm run watch

Generate standard and minified CSS assets

npm run watch -- --min

Generate standard and RTL CSS assets

npm run watch -- --rtl

Generate standard, RTL, minified and minified RTL CSS assets

npm run watch -- --min,rtl

But why split it this way?

By splitting the watch command this way we have essentially decreased the time it takes to regenerate CSS assets by 50%, so development should be faster.

Previous watch run time: 2.7s
Current watch run time: 1.34s

[richtabor]

I've updated this to make them similar to our development scripts on CoBlocks, where setup installs/sets up and start runs our development config.