fix(editor): clear selection after toggle latex editor (#12637)

- **fix(editor): clear selection after toggle latex editor**
- **chore: remove useless test**

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **New Features**
  - LaTeX rendering now outputs MathML format for improved accessibility and compatibility.
  - Added support for KaTeX styling to enhance LaTeX display in the playground.

- **Bug Fixes**
  - Improved editor behavior by resetting the selection group before opening the LaTeX editor.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/playground/package.json

@@ -24,6 +24,7 @@
     "@types/katex": "^0.16.7",
     "browser-fs-access": "^0.37.0",
     "jszip": "^3.10.1",
+    "katex": "^0.16.11",
     "lit": "^3.2.0",
     "lz-string": "^1.5.0",
     "rxjs": "^7.8.1",