Preserve multiple `Set-Cookie` headers

[jordanbtucker]

This PR ungroups Set-Cookie headers since each header should appear independently. According to MDN:

To send multiple cookies, multiple Set-Cookie headers should be sent in the same response.

For example, the following is correct:

Set-Cookie: a=b
Set-Cookie: c=d

While the following is incorrect:

Set-Cookie: a=b,c=d

This PR achieves this by:

• Adding an exported formatHeaders function in utils/misc.ts that treats set-cookie as a special case.
• Removing the private formatHeaders function from HttpResponseTextDocumentView and updating all calls to use the new utility formatHeaders function.
• Removing the private static formatHeaders function from HttpResponseWebview and updating all calls to use the new utility formatHeaders function.
• Updating getFullResponseString in HttpResponseWebview to use the new utility formatHeaders function.
• Updating createRequestInTempFile in HistoryController to use the new utility formatHeaders function.

Fixes #722