Add support for earlier OS versions + Change stream line separator to CRLF

[MMP0]

Lower the minimum supported OS versions to macOS 10.15 and iOS 13, the lowest versions that support Swift Concurrency.

Also I noticed an issue while creating a polyfill: the Twitter developer page mentions that the line separator in the streaming API is \r\n, but AsyncLineSequence interprets CR (\r), LF (\n), CRLF (\r\n), NEL (\u{85}), LS (\u{2028}) and PS (\u{2029}) as line separators. This could potentially cause problems with tweets containing these characters. So I created AsyncCRLFLineSequence.

[MMP0]

Thank you for pointing those out. I’ve fixed them.