NetConn has to close the connection to interrupt in progress reads
and writes. However, it can block reads and writes that occur
after the deadline instead of closing the connection.

Closes #228

This way we don't pollute the directory tree.

Updates #209

Closes #199

See https://github.com/nhooyr/websocket/issues/182

Closes #182

Too many improvements and changes to list.

Will include a detailed changelog for release.

- For JS we ensure we indicate which size initiated the close first from our POV
- For normal Go, concurrent closes block until the first one succeeds instead of returning early

I changed my mind after #103 as browsers include a wasClean event to indicate
whether the connection was closed cleanly. From my tests, if a server using
this library prior to this commit initiates the close handshake, wasClean
will be false for the browser as the connection was closed before it could
respond with a close frame. Thus, I believe it's necessary to fully implement
the close handshake.

@stephenyama You'll enjoy this.

Subprotocols didn't work with WASM at all before.

Closes #150

- Also allows SetReadLimit to be called concurrently which is a nice
  touch

Closes #153

Realized I can at least make the Reader/Writer/SetReadLimit methods
work as expected even if they're not perfect.

Closes #121