Support compression on text/event-stream

[nickchomey]

Currently Deno compresses responses based on this database of content types. It doesn't include text/event-stream, which is very commonly used for Server-Sent Events (which is really just a normal http response with text/string content of a specific format).

Ive successfully compressed event-stream responses with Caddy with its normal compression encoding directive. Nothing fancy needs to happen to make this work.

It would be great if Deno could support this as well, so that we don't need to implement it in the application or use a proxy like Caddy.