Refactor linting tasks to remove Gulp

[domoscargin]

What

Update tasks/gulp/lint.js to replace Gulp with our own lint scripts. There are two tasks in this file, one to lint SCSS and one to lint JS. Note that we currently spawn a process to run the linting tasks. We could probably tidy this up a bit.

Why

We have made the decision to move away from Gulp and write our own build scripts.

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• tasks/gulp/lint.js no longer sdepends on Gulp
• standard and stylelint are as up to date as feasible
• docs/contributing/tasks.md has been updated
• NPM scripts lint, lint:js and lint:scss work correctly
• lint.js has been relocated to tasks folder

[colinrotherham]

I've struck off the last "Done when" task as we no longer need a separate lint.js file for 'lint:scss'

We have a new task helper npmScriptTask() which calls npm scripts directly:

/**
 * Umbrella styles tasks (for watch)
 * Runs Sass code quality checks and compilation
 */
gulp.task('styles', gulp.series(
  npmScriptTask('lint:scss', ['--silent']),
  'scss:compile'
))

[colinrotherham]

I've unintentionally done three of the tasks (sorry) so I'll assign myself to this one

[colinrotherham]

✅ All our stylelint and standard packages are up to date too

There's one more related change so I've opened #2916