fix: avoid mangling code from incorrect magic-string usage

[benmccann]

Description

Fixes #7365

The current default in magic-string seems like a bit of a footgun. Until / unless the default is changed (Rich-Harris/magic-string#211), we should manually specify contentOnly: true

Additional context

This is causing difficult to diagnose errors in SvelteKit with SvelteKit itself giving errors like "Method not allowed" due to incorrectly compiled .js files

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[bluwy]

LGTM. I don't think there are places where we'd expect the contentOnly: false behaviour. And the tests passed too.