feat(transformer): finish 2016 exponentiation operator

[Boshen]

The bulk of the code for review: https://github.com/web-infra-dev/oxc/pull/996/files#diff-4950080c26a449198bc5b125fd72a73afe42cddde9943bc7280fa1cbb7310ced

It took me a while to understand what babel was doing ...

As a bonus, I found swc producing the wrong code for the following snippet:

let x = {};
let y = 0;
let z = 0;
x[z++] **= y;

Gets transformed into

let x = {};
let y = 0;
let z = 0;
var ref = x[z++];
x[z++] = Math.pow(ref, y);

Notice how z++ is evaluated twice.

---

Which means I should ...

• add more unit tests for this PR

And I also noticed that the benchmark for transformer is wrong ...

• Fix transformer benchmark because it is transforming the already transformed AST

[codspeed-hq]

CodSpeed Performance Report

Merging #996 will not alter performance

_{Comparing exponentiation-operator (8131529) with main (6c18b3e)}

Summary

✅ 22 untouched benchmarks

[Boshen]

I'll work on the TODOs today.