Add Babel plugin for optional chaining

What does this MR do?

add support for ?.

before: gon && gon.current_user && gon.current_user.name
now: gon?.current_user?.name

downsides

• might break people's editor syntax highlighting

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers