Responses with multiple `Content-Encoding` headers not decompressed

Responses containing multiple content-encoding headers do not fully decompress the body, resulting in an unexpected value being received.

It appears that workerd looks for an exact value match in the `content-encoding` header when deciding what to do (https://github.com/cloudflare/workerd/blob/main/src/workerd/api/system-streams.c%2B%2B#L413-L428), and as a result does not seem to support stacked headers, as they're merged into a comma-delimited list that is not walked through from what i understand.

Example reproduction in a branch in our repo - https://github.com/cloudflare/vinext/blob/codex%2Fworkerd-stacked-content-encoding-repro/repros/workerd-stacked-content-encoding/README.md

Whereas, Node will decode for both of the values and return the expected response.

This surfaced in a customer's reproduction in https://github.com/cloudflare/vinext/issues/2983

Start at src/workerd/api/system-streams.c++ around lines 413-428, where content-encoding is checked, and read the reproduction in the linked vinext README. Trace how comma-delimited encoding values are handled and verify the response body is fully decompressed for stacked encodings, matching Node's result.

Written by the indexing model from the issue text.