perf: Use optional chaining

[VIKTORVAV99]

As mentioned in #2181 this PR changes the code to use optional chaining. This PR is expected to stay open until the maintainers make a new major release that drops support for Node 12.

These changes should improve performance by reducing property access and will make the minimised code bundle smaller. These changes have not been benchmarked.

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: 96.3% (-0.01%) from 96.313%
when pulling 00ff5d5 on VIKTORVAV99:optional_chaining
into 9ce9150 on i18next:master.