Deprecate ToolbarHandler service and move toolbar integration into sub module.

This change deprecates the ToolbarHandler service in the Environment Indicator module and moves toolbar-related integration into a new optional submodule: environment_indicator_toolbar.

Key changes:

• A new submodule, Environment Indicator - Toolbar Integration, provides toolbar styling and integration functionality. It depends on the toolbar core module and the base environment_indicator module.
• The old toolbar_integration configuration setting is deprecated and no longer required. If present, it is cleared during the submodule’s installation.
• The following functions are marked as deprecated and will be removed in 5.0.0:
  • _environment_indicator_external_integration_is_enabled()
  • _environment_indicator_switcher_links()
  • _environment_indicator_switcher_cache_tags()
• The ToolbarHandler class is marked deprecated and replaced by logic inside the new submodule.
• A new JavaScript file (toolbar.js) and CSS file (toolbar.css) are introduced in the submodule.
• Toolbar icon images were moved from the base module to the submodule.
• The configuration form warns users about the deprecation and suggests enabling the new submodule instead.
• Compatibility logic remains in place in 4.x to avoid breaking existing installs that rely on the old method.

This is part 2 of a multi-part change. Previous deprecations can be tracked at:

• https://www.drupal.org/node/3526812