## Summary of the Pull Request
Originally there were 3 different methods for implementing VT cursor movement, and between them they still couldn't handle some of the operations correctly. This PR unifies those operations into a single method that can handle every type of cursor movement, and which fixes some of the issues with the existing implementations. In particular it fixes the `CNL` and `CPL` operations, so they're now correctly constrained by the `DECSTBM` margins.
## References
If this PR is accepted, the method added here should make it trivial to implement the `VPR` and `HPR` commands in issue #3428.
## PR Checklist
* [x] Closes#2926
* [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
* [ ] 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: #xxx
## Detailed Description of the Pull Request / Additional comments
The new [`AdaptDispatch::_CursorMovePosition`](d6c4f35cf6/src/terminal/adapter/adaptDispatch.cpp (L169)) method is based on the proposal I made in issue #3428 for the `VPR` and `HPR` comands. It takes three arguments: a row offset (which can be absolute or relative), a column offset (ditto), and a flag specifying whether the position should be constrained by the `DECSTBM` margins.
To make the code more readable, I've implemented the offsets using [a `struct` with some `constexpr` helper functions for the construction](d6c4f35cf6/src/terminal/adapter/adaptDispatch.hpp (L116-L125)). This lets you specify the parameters with expressions like `Offset::Absolute(col)` or `Offset::Forward(distance)` which I think makes the calling code a little easier to understand.
While implementing this new method, I noticed a couple of issues in the existing movement implementations which I thought would be good to fix at the same time.
1. When cursor movement is constrained horizontally, it should be constrained by the buffer width, and not the horizontal viewport boundaries. This is an issue I've previously corrected in other parts of the codebase, and I think the cursor movement was one of the last areas where it was still a problem.
2. A number of the commands had range and overflow checks for their parameters that were either unnecessary (testing for a condition that could never occur) or incorrect (if an operation overflows, the correct behavior is to clamp it, and not just fail). The new implementation handles legitimate overflows correctly, but doesn't check for impossible ranges.
Because of the change of behavior in point 1, I also had to update the implementations of [the `DECSC` and `CPR` commands](9cf7a9b577) to account for the column offset now being relative to the buffer and not the viewport, otherwise those operations would no longer work correctly.
## Validation Steps Performed
Because of the two changes in behavior mentioned above, there were a number of adapter tests that stopped working and needed to be updated. First off there were those that expected the column offset to be relative to the left viewport position and constrained by the viewport width. These now had to be updated to [use the full buffer width](49887a3589) as the allowed horizontal extent.
Then there were all the overflow and out-of-range tests that were testing conditions that could never occur in practice, or where the expected behavior that was tested was actually incorrect. I did spend some time trying to see if there was value in updating these tests somehow, but in the end I decided it was best to just [drop them](6e80d0de19) altogether.
For the `CNL` and `CPL` operations, there didn't appear to be any existing tests, so I added some [new screen buffer tests](d6c4f35cf6) to check that those operations now work correctly, both with and without margins.
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