Preview feature for all editors

[Myriad-Dreamin]

These are the last featured PRs in 0.12.20, to help use preview with the language server out of box, which is useful for editors other than vscode, e.g. helix and zed:

• feat: browsing preview #1234
• feat: run preview server in background #1233
• feat: CLI attach preview task running in the language server #1235
• feat: provide a sync button in the preview panel #1236

After finished them, 0.12.20-rc1 will be published for testing.