Detect VSCode theme and tell people to use a different one for better experience

[carmenberndt]

The minimal themes do not use all the root groups from the textmate grammer used for syntax highlighting, leading to a bad experience for users.

We could detect the theme and create a toast telling people about this.

[janpio]

Text suggestion:

The VSCode Color Theme foo you are using unfortunately does not fully support syntax highlighting. We suggest you switch to bar which does fully support it and will give you a better experience.