feat(data-grid): use codicon chevrons for group and tree controls

[lukecotter]

📝 PR Overview

Replaces Tabulator's stock group-header and data-tree expand/collapse controls with VS Code codicon chevrons, so the call-tree, SOQL, and DML grids match the native VS Code tree look instead of upstream's border-triangle arrows and boxed +/− buttons.

🛠️ Changes made

• Replace the group-header .tabulator-arrow border-triangle with a codicon chevron (right = collapsed, down = expanded) drawn via ::before.
• Replace the boxed +/− .tabulator-data-tree-control with the same chevrons, keeping the 16px footprint so tree indentation is unchanged.
• Hoist the shared glyph styling into a codicon-chevron mixin and the codepoints into $codicon-chevron-right/$codicon-chevron-down variables.

🧩 Type of change (check all applicable)

• 🐛 Bug fix - something not working as expected
• ✨ New feature – adds new functionality
• ♻️ Refactor - internal changes with no user impact
• ⚡ Performance Improvement
• 📝 Documentation - README or documentation site changes
• 🔧 Chore - dev tooling, CI, config
• 💥 Breaking change

📷 Screenshots / gifs / video [optional]

Will add before/after after opening.

✅ Tests added?

• 👍 yes
• 🙅 no, not needed
• 🙋 no, I need help

📚 Docs updated?

• 🔖 README.md
• 🔖 CHANGELOG.md
• 📖 help site
• 🧪 Marked any pre-release-only features (README 🧪 badge — see RELEASING.md)
• 🙅 not needed

Anything else we need to know? [optional]

TODO:

• Indent child items within grouped rows so the hierarchy reads clearly under the new chevrons.