style: introduce eslint and prettier to the webview subproject

This MR is preparing the webview subproject for further development by introducing eslint and prettier.

Up until this point, we haven't touched the webview and so the code wasn't automatically formatted. The webview component is responsible for showing "Overview" of an issue or an MR:

The MR doesn't change any production code logic except for a few places where it replaces == with ===.

Part of #215 (closed)