feat: Support for opt-in caching

[simon-paris]

This adds caching, which is off by default and enabled using {caching: "always"}. It works by mapping from snapshots provided by typescript to snapshots generated by the plugin using a WeakMap.

The caching option allows "always" | "never". I chose to use a union instead of a bool to 1) later support for a third "only when safe" or "only when supported" option, and 2) to help indicate that it might be too aggressive since "always" sounds stronger than true.

Fixes #215

cc @jsliang

[simon-paris]

Hi @mrmckeb, would you be able to review this?

[mrmckeb]

Hi @simon-paris, sorry for the long delay. I changed jobs and moved cities over the last few months and didn't get much free time for open source.

I'll take a look at this with some other changes this weekend hopefully! :)