Unicode variable names don't build

[GeorchW]

🐛 bug report

We have this library as a transitive dependency, and it won't build with parcel since it uses Unicode variable names. I can reduce the example to this:

let ü = "ü";
alert(ü);

Building this with parcel build example.js fails with this error code:

@parcel/optimizer-swc: Expected unicode escape

When starting with parcel example.js, the output in the dist folder contains these relevant lines:

let \xfc = "\xfc";
alert(\xfc);

The unicode is converted to escape sequences regardless of where it occurs in the source, which is not correct.

The bug prevents us from using Plotly with Parcel.

🎛 Configuration (.babelrc, package.json, cli command)

Clean reproduction in bash:

mkdir 2024-05-04-parcel-unicode-repro
cd 2024-05-04-parcel-unicode-repro
echo 'let ü = "ü"; alert(ü)' > repro.js
yarn add parcel
yarn run parcel build repro.js

The error message here is slightly different:

@parcel/optimizer-swc: `let` cannot be used as an identifier in strict mode

It goes away when renaming the ü to u though.

🌍 Your Environment

Software	Version(s)
Parcel	2.12.0
Node	18.20.2
Yarn	1.22.19
OS	Linux

[mischnic]

Duplicate of #9607, this will be fixed with #9574