Preserve the cursor row during Clear Buffer

[lhecker]

Introduces an ABI change to the ConptyClearPseudoConsole signal. Otherwise, we have to make it so that the API call always retains the row the cursor is on, but I feel like that makes it worse.

Closes #18732 Closes #18878

Validation Steps Performed

• Launch ConsoleMonitor.exe
• Create some text above & below the cursor in PowerShell
• Clear Buffer
• Buffer is cleared except for the cursor row ✅
• ...same in ConPTY ✅