fix(types): Accept `string` for `UserConfig.indentation`

[andersk]

Why am I submitting this PR

JSON.stringify already accepts a string here, and it’s useful to be able to pass "\t".

Does it fix an existing ticket?

No

Checklist

• [x] only relevant code is changed (make a diff before you submit the PR)
• [x] do no modify the version in package.json or CHANGELOG.md
• [ ] tests are included and pass: yarn test (see details here)
• [ ] documentation is changed or added