suggestion(http): change `ServeDirOptions.headers` to accept `HeadersInit` instead of `string[]`

[iuioiua]

It surprised me when I discovered serveDir() accepted string[] instead of HeadersInit as an option. It makes the user write needlessly write a raw header. I suggest we change this in @std/http/unstable-file-server without changing the CLI behaviour.

API reference: https://jsr.io/@std/http/doc/~/ServeDirOptions.headers