[EditContext] Prevent redundant selection synchronization to avoid conflicts

This submission primarily addresses the issue of text replacement suggestion conflicts caused by redundant selection synchronization in EditContext. The core changes include: 1) At the remote_cocoa layer, replacing the hardcoded NSSpellChecker.sharedSpellChecker with the self.spellChecker property for the spell checker; 2) At the Blink engine layer, avoiding selection synchronization from Editor when EditContext is active, as selection synchronization is handled by EditContext itself; 3) Adding a sync_selection parameter to the EditContext::SetSelection method to prevent unnecessary synchronization during temporary selection adjustments in deletion operations. These changes are mainly intended to fix the issue where suggestions are incorrectly accepted during text input.