Replace "transports" with "writers"

[alcarney]

This PR requires #507 be merged first

Now that we use the high-level asyncio APIs, it no longer makes sense for JsonRPCProtocol to be a subclass of asyncio.Protocol. This PR converts JsonRPCProtocol into a simple class, cleaning up some dead code in the process

It also

• renames the concept of "transport" to "writer" which aligns better with the reader/writer pair you often see in the high-level asyncio APIs.

• renames the connection_made method to set_writer, also incorporating an include_headers flag.
  Before

  protocol._send_only_body = True  # Don't send headers within the payload
  protocol.connection_made(WebSocketTransportAdapter(websocket))

  After

  protocol.set_writer(WebSocketWriter(websocket), include_headers=False)  

• Adds relevant typing protocols and writer classes to the pygls.io_ module

• Tweaks the handling of writers, so that both sync and async methods on the writer object can be used.

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly

Automated linters

You can run the lints that are run on CI locally with:

poetry install --all-extras --with dev
poetry run poe lint

[tombh]

So clean and new 🥹 You're really cleaning up the core.

[alcarney]

Thanks!