fix: overhaul headers proxy

[huseeiin]

this fixes many issues with the original headers proxy, for example, now:
getRequestEvent().response.headers.toString() returns "[object Headers]" instead of "[object Object]"
getRequestEvent().response.headers instanceof Headers will return true
and so on, because we're now using a native Headers object

Please check if your PR fulfills the following requirements:

• Addresses an existing open issue: fixes #000
• Tests for the changes have been added (for bug fixes / features)

What is the current behavior?

What is the new behavior?

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🦋 Changeset detected

Latest commit: c2bfa86

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@solidjs/start	Patch

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commit: c2bfa86

[huseeiin]

don't merge this, its stupid. after h3 v2 we will get a proper response Headers