accessibility_tool 2.1.0

Accessibility Tool 2.1.0 improves the visibility, configurability, and
accessibility of the tool's controls.

Changes include:

- Added optional custom colors for the tool background, control background,
control text, and control icons.
- Added configurable tool background opacity.
- Added contrast validation for control text, icons, and page contrast schemes.
- Added accessible hover, focus, active, and keyboard focus styles.
- Added an option to show text labels beside control icons.
- Improved icon-only labels for screen readers.
- Reorganized the settings page for site builders.
- Replaced theme-dependent CSS selectors with module-owned selectors.
- Added configuration schema and an update path for existing installations.
- Improved site-builder documentation.
- Added Drupal 10 and Drupal 11 kernel test coverage.

This release supports Drupal 10.2 or later in the Drupal 10 series and Drupal
11.3 or later.