Initial structure and connection to build/watch for GitLab Duo Chat

The MR creates the standard, automatically-generated Vue project (the whole webviews/gitlab_duo_chat) to serve as the Webview for GitLab Duo Chat and connects the process of building and watching this package to the overall extension's mechanism.

Since the extension is supposed to work in both: desktop and web-based versions of VSCode, the connection to GitLab VSCode Extension happens from within common/ and not browser/ or desktop/.