feat: add useAtYourOwnRisk_mutateSwcOptions option

[ArnaudBarre]

For a long time I asked people to do a patch to get access to feature I don't want to maintain (hello not yet stable decorators)

I think this option will create less issues/PRs while still having the benefit that users know they are in a non standard setup.

[patak-dev]

I think in the same way that we allow editing other low level options, we could also directly add a swcOptions config field and add the "use at your own risk" in the docs (and JSDoc). And it could be a merge like with rollupOptions. But if you think this will cause issues down the road, I'm ok with this PR (maybe renaming to useAtYourOwnRiskSwcOptions)

[ArnaudBarre]

I choose the mutate API because I don't want to have to deal with merging, it's always complex to have a good default for arrays. I can change the api to a version where you return the object, but mutating feels simpler (see the example) and it's always safe because all options a generated on every request
Having "use at your risk" only in the docs would be too easy to use IMO, I want people to remember when they read their vite config that they are using unstable decorator transform.

[patak-dev]

Sounds good to me. The edit is a bit confusing to me. Maybe useAtYourOwnRisk_mutateSwcOptions

[hi-ogawa]

Naming bike-shed aside, looks good to me.