Support parsing of SSE events

[micahhahn]

Hey @bflyblue!

Much like the author of #4 I found myself wanting to use this library to parse SSE for communicating with an LLM API.

The work to do so was not so bad - I didn't realize that #5 existed though until after it was done. Perhaps for the better since I didn't have knowledge of the pre-Servant.API.Stream API.

Some notes:

• Adding a FromServerEvent class fit well parity wise with ToServerEvent
• I made the (probably breaking) change of converting ToServerEvent to Text. This matches the specs better which explicitly specify that streams must be UTF-8.
• The spec says that newlines in the form of \r\n, \r, and \n should all be supported when interpreting a stream, so I've done that.
• I needed to expose ServerEventFraming for my use case because I'm trying to make a streaming POST request. My endpoint looks like StreamPost ServerEventFraming EventStream (SourceIO x)
• I went for the lightest touch possible changing this file, the tests from Rudimentary servant-client support and parsing of SSE events #5 are probably salvageable. We could also add roundtrip-tests around FramingRender <-> FramingUnrender and ToServerEvent <-> FromServerEvent