Implement ChainExpression type and update MemberExpression

[junoatwork]

Fixes #103

See https://github.com/estree/estree/blob/master/es2020.md#chainexpression for reference, or this really long implementation thread at estree for background on how this AST design evolved from the earlier babel efforts implemented in #98.

To implement this, I looked at the current behavior of OptionalCallExpression and OptionalMemberExpression. I was able to preserve the output of OptionalMemberExpression, but the current output for OptionalCallExpression and CallExpression are very divergent: CallExpression prints the function name, while OptionalCallExpression prints the function name and argument values.

Please advise on which approach is preferred.

This is my first PR to this repo, so please offer any change requests, and I'd be happy to make them! Thank you for your time and consideration.

[junoatwork]

Closing - see #102.

[ljharb]

Indeed, duplicate of #102 (this and that are both, unfortunately, breaking changes)