fix: ensure invalid identifiers are quoted in the props object

[cellog]

Discovered in the wild: we need to use 'quoted-strings' for props in contents that are not valid identifiers in javascript.

This fixes that (reference #1869)

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

Checklist (for documentation change)

• only relevant documentation part is changed (make a diff before you submit the PR)
• motivation/reason is provided
• commit message and code follows the Developer's Certification of Origin

[coveralls]

coverage: 97.598% (+0.005%) from 97.593%
when pulling 998f4f7 on cellog:issue-1869-quote-props-bug
into 10c2c57 on i18next:master.

[adrai]

v16.1.3