Fix transform of delete a?.b in function params

[nicolo-ribaudo]

Q	A
Fixed Issues?	Fixes #15738
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

Please review commit-by-commit.

The first one splits the existing transform logic in two parts, without causing any observable change:

1. how to, in general, traverse and transform an optional chain (moved to the transformOptionalChain function)
2. how the output should change depending on whether the optional chain is in a "read" context or in a "delete context". This will make it incredibly easy to implement a?.b = c in the future, which I'm presenting at the next TC39 meeting

The second commit actually fixes the bug, that I found while working on a?.b = c because function f(x = a?.b = c) {} was also broken.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/54745/