Allow deprecating alter functions from themes

Problem/Motivation

#2866779: Add a way to trigger_error() for deprecated hooks → added a way to deprecate alter hooks within modules. However, this capability doesn't exist in \Drupal\Core\Theme\ThemeManagerInterface making it difficult to deprecate alter functions from themes.

Proposed resolution

Add API to \Drupal\Core\Theme\ThemeManagerInterface similar to what we have in \Drupal\Core\Extension\ModuleHandlerInterface::alterDeprecated.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet