feat: Refactor Select and Updater for improved readability and functionality

Summary of changes
- Updated `select.mjs` to clarify observer context
- Refactored `updater.mjs` for cleaner change application

In `select.mjs`:
- Added a comment to indicate the observer context is a `ProxyObserver`, improving code readability and making intentions clear.
- This change helps future developers quickly understand the purpose of the code, reducing potential confusion.

In `updater.mjs`:
- Simplified the while loop condition to check `this[pendingDiffsSymbol].length` directly.
- Refactored the change application logic into a private method `#applyChange`, consolidating the steps of removing, inserting, updating content, and attributes into a single place.
- This makes the code cleaner and more maintainable, reducing duplication and enhancing readability.