This PR adds support for the `DA2` (Secondary Device Attributes) and
`DA3` (Tertiary Device Attributes) escape sequences, which are standard
VT queries reporting basic information about the terminal.
The _Secondary Device Attributes_ response is made up of a number of
parameters:
1. An identification code, for which I've used 0 to indicate that we
have the capabilities of a VT100 (using code 0 for this is an XTerm
convention, since technically DA2 would not have been supported by a
VT100).
2. A firmware revision level, which some terminal emulators use to
report their actual version number, but I thought it best we just
hardcode a value of 10 (the DEC convention for 1.0).
3. Additional hardware options, which tend to be device specific, but
I've followed the convention of the later DEC terminals using 1 to
indicate the presence of a PC keyboard.
The _Tertiary Device Attributes_ response was originally used to provide
a unique terminal identification code, and which some terminal emulators
use as a way to identify themselves. However, I think that's information
we'd probably prefer not to reveal, so I've followed the more common
practice of returning all zeros for the ID.
In terms of implementation, the only complication was the need to add an
additional code path in the `OutputStateMachine` to handle the `>` and
`=` intermediates (technically private parameter prefixes) that these
sequences require. I've done this as a single method - rather than one
for each prefix - since I think that makes the code easier to follow.
VALIDATION
----------
I've added output engine tests to make sure the sequences are dispatched
correctly, and adapter tests to confirm that they are returning the
responses we expect. I've also manually confirmed that they pass the
_Test of terminal reports_ in Vttest.
Closes#5836
53b224b1c6