WSMessage.close and WSCloseCode

[swhitty]

First requested within: #137

Adds new case to WSMessage slowing servers to send and receive custom close codes sent by clients;

case close(code: WSCloseCode = .normalClosure, reason: String = "")

Adds WSCloseCode to represent the common codes, but allowing any UInt16 to be sent.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 94.57%. Comparing base (0285d3e) to head (92d43d4).

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
FlyingFox/Sources/WebSocket/WSCloseCode.swift	50.00%	3 Missing ⚠️
FlyingFox/Sources/WebSocket/WSHandler.swift	91.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
- Coverage   94.68%   94.57%   -0.11%     
==========================================
  Files          61       62       +1     
  Lines        3985     4015      +30     
==========================================
+ Hits         3773     3797      +24     
- Misses        212      218       +6     

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