RSDK-14081: support Interactive shell on windows

[allisonschiang]

This PR replaces the unix-only pty with a charmbracelet/xpty which works on both unix and windows.

Testing:
This update is on the server-side, so I need to set up a viam-server with these updates and then shell into it
Mac -> Windows before:

viam machine part shell --part-id 123
Info: Ensure machine part has a valid shell type service
rpc error: code = Unknown desc = shell not supported on windows yet; sorry

Mac -> Windows after:

rdk git:(RSDK-14081) ./bin/Darwin-arm64/viam-cli machine part shell --part-id 123
Info: Ensure machine part has a valid shell type service

Windows PowerShell                                                                                                                                                                                      
Copyright (C) Microsoft Corporation. All rights reserved.                                                                                                                                               
                                                                                                                                                                                                        
PS C:\Users\admin\Downloads\rdk> ls                                                                                                                                                                     
                                                                                                                                                                                                        
                                                                                                                                                                                                        
    Directory: C:\Users\admin\Downloads\rdk                                                                                                                                                             
                                                                                                                                                                                                        
                                                                                                                                                                                                        
Mode                LastWriteTime         Length Name                                                                                                                                                   
----                -------------         ------ ----                                                                                                                                                   
d-----         6/5/2026   6:38 PM                .artifact        

Also confirmed shelling into mac still works through windows -> mac running this viam-server

[allisonschiang]

windows doesn't take the -i (interactive shell) or term input using nil here still works

[allisonschiang]

we pass the ctxCancel into xpty.WaitProcess on line 134 because exec can't launch a conPTY process

see: golang/go#62708