task: add root tsconfig

[timReynolds]

As mentioned in this comment as part of #668 it would be better to extend a core tsconfig file than duplicate the settings in every package.

This PR introduces that change placing the core settings in a new tsconfig.json at the root level and then extended everywhere else.

As strict mode was set in most packages I've included it in the root settings and disabled it the two places (core & css) which previously didn't have it set. You'll probably want to fix these packages in separate PRs.

I wasn't clear if the intent was to have to root file called .tsconfig.json as mentioned in the comment, it didn't feel like the convention so it's currently tsconfig.json happy to change it if you'd like.

I wasn't sure how to test this, test don't seem to be working, so I executed yarn prepare which passes in all packages.

[mxstbr]

Amazing, thank you so much for doing this! 💯 This looks great