Add support for vscode.dev and other web vs code editions

[vbifonixor]

Hi!
The extension works really good, but it would be even nicer if I could install it on vscode.dev or github codespaces to edit beancount files from within browser.

[mengqi92]

The extension currently relies on running beancount command locally to provide diagnostics. I'm not sure if there is a way to properly support this in the browser.

[vbifonixor]

I think that it should be possible to make a subset of plugin's functionality available for browser environments, at least syntax highlighting