Move off TSLint

[coolsoftwaretyler]

We are still using TSLint, which has been deprecated for five years, so we should update to typescript-eslint.

[alyssadicarlo]

I’ve been in touch with Tyler about this issue and would like to take it on. We’ve discussed the plan to wait until the Prettier formatting changes (from #2226) are merged before committing anything major, but in the meantime, I’ll begin researching and spiking out the migration from TSLint to typescript-eslint.