Add support for the DECRQCRA checksum report

[j4james]

This PR implements the DECRQCRA escape sequence, which lets you
request a checksum of a portion of the screen. This is most useful in
automated testing to verify that the generated screen content is what it
was expected to be.

For now this functionality is gated behind a feature flag which is only
enabled for dev builds.

Detailed Description of the Pull Request / Additional comments

I've done my best to match the DEC checksum algorithm as closely as
possible, which we've determined by testing on a real VT525 terminal
(many thanks to @al20878 for that).

The checksum is an unsigned 16-bit value that starts off at zero, and
from which you then subtract the ordinal value of every character in the
selected range. It's also affected by the rendition attributes in the
selected cells.

• Bold/Intense - subtract 0x80
• Blinking - subtract 0x40
• Reverse video - subtract 0x20
• Underlined - subtract 0x10
• Invisible - subtract 0x08
• Protected - subtract 0x04
• Background color - subtract the color index
• Foreground color - subtract the color index * 0x10

I should note that our ordinal calculation only matches DEC for the
characters in the ASCII and Latin-1 range, because the original
algorithm predates Unicode. If we want to support the other character
sets correctly we'll need custom mapping tables, but I didn't think that
was essential for now.

It's also worth mentioning that we don't handle "empty" cells correctly,
but that's not the fault of the checksum calculation - it's just that
our default fill character is a space rather than a NUL.

Validation Steps Performed

I've manually compared our implementation against the tests results that
@al20878 got from the VT525, and confirmed that we match as well as was
expected (i.e. taking into account the limitations mentioned above).

I've also added a few basic unit tests that verify we're generating the
expected checksums for the various renditions and color attributes.

Closes #14974

[j4james]

Not sure if the build is failing is because I've done something wrong with the feature flag configuration, or it's just one of the usual CI failures. But as far as I can see, it's complaining about a circular dependency in TerminalAzBridge.vcxproj, which I don't think is related to anything I've changed.

[zadjii-msft]

Yea that looked like another #14581 hit to me. Weird that it hit there, but 🤷

[zadjii-msft]

I think it's really impressive how this all came together. More than happy to check this in as a debugging tool ☺️

[DHowett]

This is a joy to behold!