Support snippet cursor tab stops

[apples-kksk]

Description

Add snippet placeholder tab-stop navigation for snippet notes.

This PR:

• Adds support for standalone $ snippet cursor placeholders.
• Adds support for 710321 as a final tab stop.
• Preserves the existing :{} cursor placeholder behavior.
• Tracks snippet tab stops with CodeMirror bookmarks so later stops stay accurate after typing.
• Gives active snippet tab-stop navigation priority before normal Tab indentation.

Validation note: ./node_modules/.bin/eslint browser/lib/SnippetManager.js browser/components/CodeEditor.js was not run because dependencies are not installed in this checkout.

Issue fixed

Fixes #2916.

Type of changes

• ⚪ Bug fix (Change that fixed an issue)
• ⚪ Breaking change (Change that can cause existing functionality to change)
• ⚪ Improvement (Change that improves the code. Maybe performance or development improvement)
• 🔘 Feature (Change that adds new functionality)
• ⚪ Documentation change (Change that modifies documentation. Maybe typo fixes)

Checklist:

• 🔘 My code follows the project code style
• ⚪ I have written test for my code and it has been tested
• ⚪ All existing tests have been passed
• ⚪ I have attached a screenshot/video to visualize my change if possible
• 🔘 This PR will modify the UI or affects the UX
• 🔘 This PR will add/update/delete a keybinding