Stricter type checking and eslint config

[aslakhellesoy]

Description

Add stricter type checking and eslint config

Motivation & context

It makes the code more type-safe, both internally and at the API level

Type of change

• Refactoring/debt (improvement to code design or tooling without changing behaviour)
• Breaking change (will cause existing functionality to not
  work as expected)

Remember to add an entry to the relevant section of CHANGELOG.md as part of this pull request.

Note to other contributors

If your change may impact future contributors, explain it here, and remember to update README.md and CONTRIBUTING.md accordingly.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[aslakhellesoy]

There are some breaking changes in here - we need to update the changelog