feat: Add WorkingDirectory property to control terminal process startup directory (WPF)

[hetima]

Summary

This PR adds a WorkingDirectory property to the EasyTerminalControl class, allowing users to specify the working directory for the terminal process. This feature enables terminal sessions to start in a specific directory instead of inheriting the current process's working directory.

Changes

• Added WorkingDirectory dependency property to EasyTerminalControl
• Updated IProcessFactory.Start() interface to accept an optional workingDirectory parameter
• Updated ProcessFactory.Start() and RunProcess() to pass working directory to CreateProcess()
• Updated TermPTY.Start() method to accept and pass through the working directory parameter
• Updated example application (TermExample) to demonstrate the new property

Details

The implementation includes fallback logic:

• If the specified directory does not exist, set it to null.
• If WorkingDirectory is null, the process inherits the current process's working directory (default behavior)

Usage Example

<term:EasyTerminalControl 
    StartupCommandLine="pwsh.exe" 
    WorkingDirectory="{Binding WorkingDirectory}" />

public string WorkingDirectory => Environment.GetFolderPath(Environment.SpecialFolder.UserProfile);

[mitchcapper]

@hetima thanks this PR looks clean. I think if the WD doesn't exist or isn't specified we should just make sure it is set to null so the behavior is the default. Let me know your thoughts.

[hetima]

I have no objection. Changes committed.