[Feat]: add support for --disable-extensions

## What is your suggestion?
- add support for --disable-extensions to `code-server`

## Why do you want this feature?
I work at a company that bootstraps development environments with vscode. Our customers are security conscious and have found that certain extensions contain security vulnerabilities. We want to permanently disable these extensions.

## Are there any workarounds to get this functionality today?
Not that I know of

## Are you interested in submitting a PR for this?
Yes