## 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
* [x] Closes#6703
* [x] CLA signed.
* [x] Tests added/passed
* [ ] Documentation updated.
* [ ] Schema updated.
* [x] I've discussed this with core contributors already. Issue number where discussion took place: #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.
7d677c5511