feat(vscode): add chat panel

[wwayne]

Screen record

https://jam.dev/c/90c63fbe-4b83-49e0-9515-1040e284b352

Local testing

you might need to run pnpm build in the root folder first

UX bug (update: solved)

1. don't open sidebar
2. in the editor, select a piece of codes, right click and select tabby:explain this
3. you will see the sidebar opened
4. but you will see the initial page first, then it start paste your codes and start chatting (I've already asked @liangfung help to add a new props to the Chat component so I can handle such case better from vscode side)

Style syncing still has issue in a edge case (update: solved)

[liangfung]

@wwayne see pr #2235
Check if this is convenient enough, calling sendUserChat within the onLoaded callback. If not feasible, consider adding a parameter like initialUserMessage to the Chat component.