Feature request: Optional chaining operator

Steps to reproduce

esprima.parse('foo?.bar')

Expected output

Some indication that this is optional chaining (if foo exists, it's the same as foo.bar, else it's undefined)

Actual output

"Unexpected token ."

Relevant references

https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#prod-OptionalExpression

This is supported in all major browsers and Node.js.