Optimize TS enums output

[liuxingbaoyu]

Q	A
Fixed Issues?	Fixes #9268
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	√
Documentation PR Link
Any Dependency Changes?
License	MIT

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/54077/

[nicolo-ribaudo]

This IIFE always returns undefined, it should return the enum.

[liuxingbaoyu]

oops! Thanks!

[nicolo-ribaudo]

Can we always use the new helper, but keeping the var declaration separate, and only adding the PURE comment when it's pure? It would make the plugin logic easier to follow, since:

1. There is only one helper
2. The "is this pure" logic is completely separated from the "have we already seen this" logic

i.e. the pure output would be

var Test;
Test = /*#__PURE__*/(function (Test) {
    Test[Test["Key"] = 1] = "Key";
    return Test;
})(Test || {});

and the not pure output would be

var Test;
Test = (function (Test) {
    Test[Test["Key"] = 1] = "Key";
    return Test;
})(Test || {});

[liuxingbaoyu]

This looks really nice, but terser seems to only remove unused var xxx = y.

[nicolo-ribaudo]

I was going to submit a review after testing locally, but I ended up having a complete PR targeting this branch 😅
liuxingbaoyu#5

It's just a refactoring so feel to merge this as-is and I can move that PR to this repo, or to merge that PR into this before merging.