Allow targets to be disabled in project.yml

[grasmash]

# Control which targets are run by the *:build:validate:test processes. You may
# still execute any target individually.
targets:
  frontend:
    build: true
  tests:
    behat: true
    phpunit: true
    security-updates: true
  validate:
    composer: true
    lint: true
    phpcs: true
    twig: true

[grasmash]

This would require some target renaming, but is more organized.

targets:
  frontend:
    build: true
  setup:
    behat: true
    drupal:
      config-import: true
      hash-salt: true
      install: true
    git-hooks: true
  tests:
    behat: true
    phpunit: true
    security-updates: true
  validate:
    composer:
      lint: true
    php:
      lint: true
      sniff: true
    twig:
      lint: true
    js:
      sniff: true