Summary of the Pull Request

This PR adds support for the SGR 2 escape sequence, which enables the
ANSI faint graphic rendition attribute. When a character is output
with this attribute set, it uses a dimmer version of the active
foreground color.

PR Checklist

• Closes Add support for SGR 2 Faint #6703
• CLA signed.
• Tests added/passed
• Documentation updated.
• Schema updated.
• I've discussed this with core contributors already. Issue number where discussion took place: Add support for SGR 2 Faint #6703

Detailed Description of the Pull Request / Additional comments

There was already an ExtendedAttributes::Faint flag in the
TextAttribute class, but I needed to add SetFaint and IsFaint
methods to access that flag, and update the SetGraphicsRendition
methods of the two dispatchers to set the attribute on receipt of the
SGR 2 sequence. I also had to update the existing SGR 22 handler to
reset Faint in addition to Bold, since they share the same reset
sequence. For that reason, I thought it a good idea to change the name
of the SGR 22 enum to NotBoldOrFaint.

For the purpose of rendering, I've updated the
TextAttribute::CalculateRgbColors method to return a dimmer version of
the foreground color when the Faint attribute is set. This is simply
achieved by dividing each color component by two, which produces a
reasonable effect without being too complicated. Note that the Faint
effect is applied before Reverse Video, so if the output it reversed,
it's the background that will be faint.

The only other complication was the update of the Xterm256Engine in
the VT renderer. As mentioned above, Bold and Faint share the same
reset sequence, so to forward that state over conpty we have to go
through a slightly more complicated process than with other attributes.
We first check whether either attribute needs to be turned off to send
the reset sequence, and then check if the individual attributes need to
be turned on again.

Validation

I've extended the existing SGR unit tests to cover the new attribute in
the AdapterTest, the ScreenBufferTests, and the VtRendererTest,
and added a test to confirm the color calculations when Faint is set
in the TextAttributeTests.

I've also done a bunch of manual testing with all the different VT color
types and confirmed that our output is comparable to most other
terminals.