Create directly_addressed Todos when mentioned in beginning of a line

app/helpers/todos_helper.rb

app/models/concerns/mentionable.rb

app/models/directly_addressed_user.rb

app/models/todo.rb

app/services/todo_service.rb

changelogs/unreleased/24976-start-of-line-mention.yml

lib/banzai/querying.rb

lib/banzai/reference_extractor.rb

lib/banzai/reference_parser/base_parser.rb

...banzai/reference_parser/directly_addressed_user_parser.rb

lib/gitlab/reference_extractor.rb

spec/factories/todos.rb

spec/lib/gitlab/reference_extractor_spec.rb

spec/services/todo_service_spec.rb