feat: Add UnwrapScalar to ShellVariablesPreferences

- Add UnwrapScalar boolean field to ShellVariablesPreferences struct.
- Initialize UnwrapScalar to false in NewDefaultShellVariablesPreferences.
- This preference will control whether shell output should be quoted or raw.

pkg/yqlib/shellvariables.go

@@ -2,11 +2,13 @@ package yqlib
 
 type ShellVariablesPreferences struct {
 	KeySeparator string
+	UnwrapScalar bool
 }
 
 func NewDefaultShellVariablesPreferences() ShellVariablesPreferences {
 	return ShellVariablesPreferences{
 		KeySeparator: "_",
+		UnwrapScalar: false,
 	}
 }