Epic: Re-implement CiviCRM Standalone (CMS-less CiviCRM)

Follow-up to #1153 (closed), and more context in this wiki page, this is an epic to track the main tasks to implement CiviCRM Standalone.

• Base classes (Hook, System, Permission and css/tpl so that Standalone can boot / proof-of-concept PR22227
• Basic composer project to handle the code-case (see: https://github.com/civicrm/civicrm-standalone)
• Composer template project that follows best-practices (see: https://github.com/civicrm/civicrm-core/pull/26771)
• Implement a breadcrumb (see: https://github.com/civicrm/civicrm-core/pull/26782)
• Login page (standaloneusers ext now included in core)
• #4053 (closed) User management and permissions
• initial user roles #4466 (closed)
• Password reset by email PR28505, PR27681
• Clean URLs
• Language switcher - language links are missing lcMessages=xx_YY and session change is not sticky. See #4425 (closed) and PR#27040
• GenCode
• Installer (using 'civicrm-setup')
• Buildkit (mostly, requires cv support, see: https://github.com/civicrm/civicrm-buildkit/pull/662)
• cv (workaround: CIVICRM_SETTINGS="/var/www/standalone/data/civicrm.settings.php" cv sql:cli)
• Backend themes (test known themes)
• Releaser support (create tar.gz for official downloads) - https://github.com/civicrm/civicrm-core/pull/27104
• Demo site (https://github.com/civicrm/civicrm-buildkit/pull/794)
• Installation documentation https://docs.civicrm.org/installation/en/latest/standalone/
• Getting started dashlet - alert.civicrm.org needs to be updated to understand "uf=Standalone" since it currently gives error 500, e.g. https://alert.civicrm.org/welcome?prot=1&ver=5.46.alpha1&uf=Standalone&sid=blah&lang=en_US&co=
  • infra/community-messages!2 (merged)
• High-level error handler. https://github.com/civicrm/civicrm-core/pull/26965
• How to treat cms.root. The rest of the work so far is treating "standalone" as if it were a cms, so it should probably return something for it.
• Nicer Role/Permission UI, permission for adding users

Known minor bugs:

• On the /civicrm/admin page, the links are missing the leading /
• Because CRM/Utils/System/Standalone.php automatically sets userID = 1, that CID is the default org record, so some public forms such as Event Registrations, behave incorrectly. Will be fixed when we implement users.
• Might be a local issue but I [DaveD] get the old files system status warning. There's an open ticket somewhere about this happening for drupal 8 sometimes so might be similar.

Nice to haves, but not release-critical:

• Front-end theme?