refactor(types): add tests and linting

[fdebijl]

This PR adds some much needed quality control to the types by adding tests, a linter and automatic checks on each PR.

Full changelog:

• Move index.d.ts to 'types' directory
• Move tsconfig.json to 'types' directory
• Add dtslint as a devdependency
• Add test.ts
  This file contains code that is not meant to be ran, but is used by dtslint to check for any errors in the types
• Add tslint.json to 'types' directory
• Add checks.yml to github actions directory
  This runs the tests, lint and dtslint on any pull request
• Fix redundant jsdoc comments in index.d.ts
• Fix extra whitespace in index.d.ts
• Fix redundant public statements in index.d.ts
• Add export statements in index.d.ts. There were implicit before but are now explicitly added for clarity.

[peterpme]

💯 this is great! once these tests pass I'll merge this in :)

[fdebijl]

@peterpme I should note that some of the problems in the typings are already adressed by other PR's, such as #113 and #139. If those are tested and merged first there should be no more errors with the types.

Edit: I can confirm #105 also fixes the issue mentioned in that PR

[fdebijl]

Closing this for now to keep my PR overview clean, feel free to re-open once this package is being worked on again 👌