When upgrading to v15, I discovered some issues with `useMeasure` forcing me to downgrade to v14 again. Hopefully these changes fixes those issues...
* It was no longer possible to define the ref type, meaning components requiring a certain type of ref would reject it. Fixed this by making the function generic again.
* Previously you _had to_ define the ref type, while in v15 you _couldn't_. Fixed this by giving the generic type a default type of `HTMLElement`.
* The implicit typing of `useMeasureMock` combined with the ternary default export statement made the imported type of `useMeasure` very messy. Fixed this by making the type of `useMeasureMock` explicitly match `useMeasure`.
chore: move all the tests to the separate directory outside of sources;
chore: remove jest.config.js (config moved to the package.json);
test: unused import in test;
test: 💍 fix tests add back x and y to useMeasure
chore: 🤖 add linter to /tests folder
ci: 🎡 limit Jest worker count for CircleCI
