Improve testability

[yamadashy]

Improve Testability by Separating DOM Operations from ChannelGrouper

Purpose

This PR aims to enhance the testability and maintainability of our Slack Channels Grouping extension by separating DOM manipulation logic from the core grouping logic.

Changes

1. Introduced a new ChannelManipulator interface to abstract DOM operations.
2. Created DomChannelManipulator to handle DOM-specific operations.
3. Refactored ChannelGrouper to use the ChannelManipulator interface, removing direct DOM dependencies.
4. Implemented a MockChannelManipulator for testing purposes.
5. Updated ChannelGrouper to handle prefix extraction, which was previously done in DomChannelManipulator.