chore: upgrade to `eslint` v6

[G-Rath]

What did you implement:

Upgrades to the next major version of ESLint (v6)

(I was originally upgrading @typescript-eslint too, but v3 returns a lot of warnings which is fine but ultimately the fact that ESLint v6 works without issue means might as well land this now)

How did you implement it:

Updated the major version in package.json, installed, and ran npm run lint.

How can we verify it:

Check if CI passes

Todos:

Note: Run npm run test:ci to run all validation checks on proposed changes

• [x] Ensure there are no lint errors.
  Validate via npm run lint
  Note: Some reported issues can be automatically fixed by running npm run lint:fix
• [x] Write tests and confirm existing functionality is not broken.
  Validate via npm test
• [x] Write documentation
• [x] Provide verification config / commands / resources
• [x] Enable "Allow edits from maintainers" for this PR
• [x] Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO