Bug 32609: Improve yarn build commands

Bug 32609: Improve yarn build commands

We had:
yarn css # Build staff css
yarn css --view opac # Build OPAC css
yarn build # Nothing, was broken
yarn watch # Nothing, was broken
yarn build_js # Build JS/Vue
yarn watch_js # Watch JS/Vue

This patch is suggesting to have something more consistent:
yarn css:build      # Build CSS for OPAC and staff (dev)
yarn css:build:prod # Build CSS for OPAC and staff (prod)
yarn css:watch      # Watch CSS for OPAC and staff (dev)
yarn js:build       # Build JS (dev)
yarn js:build:prod  # Build JS (prod)
yarn js:watch       # Watch JS (dev)
yarn build          # Same as css:build and js:build (dev)
yarn build:prod     # Same as css:build:prod and js:build:prod (prod)

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>