Bug: Login non functional in Theia

Login is implemented using GiuHub Oauth 2.0.
It is working when launching the extension in vscode but not with Theia, as the register of Uri handler is not supported, see [theia issue](https://github.com/eclipse-theia/theia/issues/13169).

This is currently handled with an hard-coded fix in the source code (user has to provide his access token [here](https://github.com/eclipsesource/ide-ai-assistant/blob/83539ba60f2aa967bb6aeecbf8738faaf634af9f/extensions/ai-assistant-vsc/src/resources/loginScript.js#L23)), which should be removed once the issue mentioned above is merged.