Where can I find a list of all possible keys for tm-themes for syntax highlighting?

tmLanguage scope names are based on convention, so there's no definitive list. The "Naming Conventions" section at the bottom of this TextMate Manual page is fairly comprehensive though.

You can also check what scopes are highlighted by VSCode's default themes, see for instance dark_plus.json and dark_vs.json (which the former is based on / includes).

Finally, as of VSCode 1.9.0, there is a builtin command to inspect tmLanguage scopes (Developer: Inspect Editor Tokens and Scopes). It will give you a lot of information over how the token at the cursor location is scoped / highlighted:

There is also a Scope Info extension which does a very similar thing, but on hover (which I personally prefer usability-wise). However, it only lists the scope names, not including any of the additional info that VSCode's built-in inspector has.