Add helpers for Nightwatch tests to install modules, create users, etc

Added API endpoints and helper commands for Nightwatch:

• thCreateUser(): Creates a new user, with optionally adding permissions and log-in.
• thLogin(): Performs a login for the given user.
• thInstallModules(): Installs one or several modules at once, optionally with dependencies.
• thUninstallModules(): Uninstalls one or several modules at once.
• thSetEnvs(): Sets the environment variables on the Drupal side.
• thGetEnv(): Gets the environment variable value from the Drupal side.

Those functions perform actions on the Drupal side, and work many times faster, than Drupal Core alternatives (drupalInstall(), drupalCreateUser(), etc).

See the usage examples in the included self-tests in the directory tests/modules/test_helpers_functional/tests/src/Nightwatch/Tests.