Allowing transpilers (like babel) to be used in templates

[chopfitzroy]

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As per this comment the ability to allow tools like babel to manipulate template code.

I am not event sure if this is in the right scope for this project but up until I started experimenting I thought issues like #3388 were actually going to allow me to use babel in my templates (I was hoping to enable optional chaining hence the above linked comment).

I wanted to open this request because I thought even if this is outside of the scope of the project it would be nice if there was an official line on why this isn't/won't be happening and then this can request can be closed.

Describe the solution you'd like
I would either like transpilers like babel to be supported in templates, or documentation added to the project explaining that this is not possible currently and is not planned (to help reduce confusion).

Describe alternatives you've considered
Described above, the alternative would be an addition to the documentation.

How important is this feature to you?
Note: the more honest and specific you are here the more we will take you seriously.

Not important, I am perfectly happy with Svelte not supporting transpilers as it does seem like it would be outside of the current scope of the project, what I would have liked was some easy to find documentation explaining that this can't be done.

Additionally to echo #3388 it would be good to have an official line adding babel to projects trying to support old browsers.

Additional context
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[Conduitry]

Duplicate of #4701.