Hardcoded 5 second timeout for writing pong response can cause errors

An inbound ping has a hardcoded 5 second deadline to handle the incoming message and send out a pong response. The general codepath for such a scenario is:
`Conn.readLoop`->`h.opcode==opPing`->`handleControl(..., opPing)`->`writeControl(..., opPong)`

The two 5 second context-based timeouts declared here:
1. https://github.com/coder/websocket/blob/9f473adfa59519d1e0295cd7a988d965085b701a/read.go#L302
2. https://github.com/coder/websocket/blob/9f473adfa59519d1e0295cd7a988d965085b701a/write.go#L277

These timeouts can cause the Websocket connection to close with an unrecoverable error. I can go into more detail about the two possible situations that can cause this condition if required, but just note that the network connection is extremely poor both in reliability and speed, and the connection is generally busy sending data (chunked enough to allow the incoming ping frames to be handled for the average network speed assuming no interruptions).

```failed to get reader: failed to handle control frame opPing: failed to write control frame opPong: failed to acquire lock: context deadline exceeded```

The problem mostly arises when the connection is briefly interrupted or the write mutex is otherwise busy underneath an in-flight inbound ping message. This 5 second timeout is not enough to be able to write the pong response.

Proposals:
1. Allow this timeout value to be configurable - at the very least for ping/pong control frames.
2. Increase this hardcoded timeout to 20 seconds by default - perhaps only for ping/pong frames?
This is generally more in line with other implementations I've found that have a default value - though most do not have a default timeout at all and rely on the user to implement ping handling in their own way.
- https://websockets.readthedocs.io/en/stable/reference/asyncio/client.html#websockets.asyncio.client.connect - `open_timeout=10, ping_interval=20, ping_timeout=20, close_timeout=10`
- https://socket.io/docs/v4/server-options/#pingtimeout (granted this isn't a pure Websocket library)

Definitely open to other ideas, and I'm happy to get a PR up for this, but for now I've forked the repo to change this value.

Start with the inbound ping flow in read.go around the opPing handler and the pong write path in write.go around the hardcoded five-second contexts. Reproduce or reason through a blocked write mutex and interrupted connection, then decide how the ping/pong timeout should behave. Done means the connection no longer fails prematurely in the reported conditions and the selected default or configuration is covered by tests.

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