chore(deps): bump diff and tslint

Bumps [diff](https://github.com/kpdecker/jsdiff) and [tslint](https://github.com/palantir/tslint). These dependencies needed to be updated together.

Updates `diff` from 3.5.0 to 4.0.4
- [Changelog](https://github.com/kpdecker/jsdiff/blob/master/release-notes.md)
- [Commits](kpdecker/jsdiff@v3.5.0...v4.0.4)

Updates `tslint` from 5.12.1 to 5.20.1
- [Release notes](https://github.com/palantir/tslint/releases)
- [Changelog](https://github.com/palantir/tslint/blob/master/CHANGELOG.md)
- [Commits](palantir/tslint@5.12.1...5.20.1)

---
updated-dependencies:
- dependency-name: diff
  dependency-version: 4.0.4
  dependency-type: indirect
- dependency-name: tslint
  dependency-version: 5.20.1
  dependency-type: indirect
...

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Author: dependabot[bot]