Provide plugin and theme directories as built-in variables #381

Scenarios run wp plugin path or wp theme path and save the output only to get the plugins or themes directory, for example as {PLUGIN_DIR} or {THEME_DIR}. These locations are known as soon as WordPress is installed, yet getting them costs a WordPress bootstrap each time. This happens about 80 times per suite run, mostly in scaffold-command and extension-command.

Providing these directories as built-in variables would remove the extra process and the boilerplate steps.