Teach PSReadline to not force the background color during render

[DHowett]

PR Summary

Upcoming versions of the Windows Console will be able to differentiate
a color set by the Win32 API from a color set by VT. This code
transforms a Win32 color into a VT color, and then uses that VT color
during rendering. When an enlightened console host receives this VT, it
may handle it differently (translate it differently, reverse it
differently, store it differently).

I've switched the defaults over to "39;49" (the well-supported SGR
codes for "foreground, background to default") and taught the two
console translators how to handle them. I went back to the Windows
source code and confirmed: conhost supports 39;49 everywhere
ENABLE_VIRTUAL_TERMINAL_PROCESSING is supported.

The first commit that introduced 39/49 is in Windows 10 TH2, the very
same release that introduced VT handling.

There's a lot more information in #830 (comment)

Fixes #830.
Related to microsoft/terminal#5952

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure you've added one or more new tests (tests updated to handle this)
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed at PowerShell-Docs
    • Doc Issue filed:

Microsoft Reviewers: Open in CodeFlow

[DHowett]

Alright, I went with a different approach. SGR 39 and 49 exist to restore the console color to the default. I've taught the mock and legacywin32 consoles how to handle this (in the same way as the windows console handles it), and switched the "reset to the default colors" sequence to set 39;49.

This does change the behavior of Render. Where it would previously enforce the current colors, it now prefers the default colors. This does not appear to be true; I cannot get PSReadline (before my change) to respect the active colors.

For what reasons were we keeping the "current" colors? Are they incontrovertible?

[DHowett]

Tested in the legacy console:

[daxian-dbw]

@DHowett Thanks for the contribution!