Convert run-script-on-filetype-change to new Webpack 4 plugin api.

[illepic]

Work in progress. No need to review yet.

Changes

• Moving our initial PL compile to webpack.pl.shared.js via simple spawnSync() scripts
• As such, removing:
  • dev:pl:webpack
  • dev:pl:gulp
  • dev:drupal:webpack
  • dev:drupal:gulp
  • dev:grav:webpack
  • dev:grav:gulp
• Any gulp tasks can be run directly via npx gulp task-name.
• run-script-on-filetype-change has been updated to Webpack 4 API.
• No longer generating source/_data/env.json on every PL compile, thus reducing Webpack emits and unnecessary writes to within source/.
• Wrote our own RunScriptAfterEmit so there is no longer a dependency on WebpackShellPlugin (which is not Webpack 4 compatible)

[illepic]

Noting that we need some big updates to the official docs. Leaning heavily on npx now to run gulp tasks.