Add types

Per discussion on the Discord I'd like to add a types for this class so that it can be properly use in a typescript project. See [this issue](https://github.com/enhance-dev/enhance-starter-typescript/issues/2) for how it is currently broken.

To do this I need to add an index.d.ts file to:

- [x] base-element
- [ ] template-mixin
- [ ] custom-elelment-mixin
- [ ] custom-elelemt

We will need to merge base-element and push it to npm before any of the others since template-mixin type is dependent upon it. We can then merge and push template-mixin and custom-element-mixin. After which we can finally merge and update custom-element.

I'll add simple tests using tsd to ensure that if the underlying classes change shape in a critical way, the type tests will fail.

I'll link the PRs in the other repos back to this issue.