Command-line-api: extract a new package for lower-level parts of System.CommandLine.Rendering

[jonsequitur]

Some portions of System.CommandLine.Rendering are currently more usable than others. A standalone package containing these simpler, more stable pieces could be useful and could come out of preview much sooner.

Those pieces include the TextSpan classes, the TextSpanParser and the ANSI escape sequence helpers.

the ITerminal, Region-based rendering, ConsoleRenderer, ScreenView, and terminal detection pieces are less ready for stabilization.

Feedback and naming suggestions for the new package welcome.

[jonsequitur]

@SteveL-MSFT @Keboo @KathleenDollard

[SteveL-MSFT]

System.CommandLine.Rendering.Core seems reasonable to me