## Summary of the Pull Request
This PR adds support for the core VT52 commands, and implements the `DECANM` private mode sequence, which switches the terminal between ANSI mode and VT52-compatible mode.
## References
PR #2017 defined the initial specification for VT52 support.
PR #4044 removed the original VT52 cursor ops that conflicted with VT100 sequences.
## PR Checklist
* [x] Closes#976
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [x] Tests added/passed
* [ ] Requires documentation to be updated
* [x] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #2017
## Detailed Description of the Pull Request / Additional comments
Most of the work involves updates to the parsing state machine, which behaves differently in VT52 mode. `CSI`, `OSC`, and `SS3` sequences are not applicable, and there is one special-case escape sequence (_Direct Cursor Address_), which requires an additional state to handle parameters that come _after_ the final character.
Once the parsing is handled though, it's mostly just a matter of dispatching the commands to existing methods in the `ITermDispatch` interface. Only one new method was required in the interface to handle the _Identify_ command.
The only real new functionality is in the `TerminalInput` class, which needs to generate different escape sequences for certain keys in VT52 mode. This does not yet support _all_ of the VT52 key sequences, because the VT100 support is itself not yet complete. But the basics are in place, and I think the rest is best left for a follow-up issue, and potentially a refactor of the `TerminalInput` class.
I should point out that the original spec called for a new _Graphic Mode_ character set, but I've since discovered that the VT terminals that _emulate_ VT52 just use the existing VT100 _Special Graphics_ set, so that is really what we should be doing too. We can always consider adding the VT52 graphic set as a option later, if there is demand for strict VT52 compatibility.
## Validation Steps Performed
I've added state machine and adapter tests to confirm that the `DECANM` mode changing sequences are correctly dispatched and forwarded to the `ConGetSet` handler. I've also added state machine tests that confirm the VT52 escape sequences are dispatched correctly when the ANSI mode is reset.
For fuzzing support, I've extended the VT command fuzzer to generate the different kinds of VT52 sequences, as well as mode change sequences to switch between the ANSI and VT52 modes.
In terms of manual testing, I've confirmed that the _Test of VT52 mode_ in Vttest now works as expected.
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