Feat: [Regions] allow setting content and id

[katspaugh]

Short description

Resolves #3124

Implementation details

Allows setting region's content and id via region.setOptions.
E.g.

region.setOptions({
  id: 'new-id',
  content: 'New content'
})

[github-actions]

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Type	Occurrences	Fixable
Errors	0	0
Warnings	0	0
Ignored	1	N/A

• Result: ✅ success
• Annotations: 0 total

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^{Report generated by eslint-plus-action}

[cloudflare-workers-and-pages]

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