wasm: support wasi console redirect for the ghci browser mode

This commit adds optional support for redirecting wasi console
stdout/stderr back to the host when running wasm ghci browser mode. By
default, the wasi console outputs are only available under F12
devtools console, but in case of testing against a mobile browser, the
devtools console may not be readily available, and it would be more
convenient to at least get wasi console output on the host side.

The redirection logic is simple, just adding another two WebSockets
connections that pump the line-buffered textual messages back to
host.