Add --shell-key-separator flag for customizable shell output format

- Add ShellVariablesPreferences struct with KeySeparator field (default: '_')
- Update shellVariablesEncoder to use configurable separator
- Add --shell-key-separator CLI flag
- Add comprehensive tests for custom separator functionality
- Update documentation with example usage for custom separator

This feature allows users to specify a custom separator (e.g. '__') when
outputting shell variables, which helps disambiguate nested keys from
keys that contain underscores in their names.

Example:
  yq -o=shell --shell-key-separator='__' file.yaml

Fixes ambiguity when original YAML keys contain underscores.

pkg/yqlib/encoder_shellvariables.go

@@ -12,10 +12,13 @@ import (
 )
 
 type shellVariablesEncoder struct {
+	prefs ShellVariablesPreferences
 }
 
 func NewShellVariablesEncoder() Encoder {
-	return &shellVariablesEncoder{}
+	return &shellVariablesEncoder{
+		prefs: ConfiguredShellVariablesPreferences,
+	}
 }
 
 func (pe *shellVariablesEncoder) CanHandleAliases() bool {
@@ -58,7 +61,7 @@ func (pe *shellVariablesEncoder) doEncode(w *io.Writer, node *CandidateNode, pat
 		return err
 	case SequenceNode:
 		for index, child := range node.Content {
-			err := pe.doEncode(w, child, appendPath(path, index))
+			err := pe.doEncode(w, child, pe.appendPath(path, index))
 			if err != nil {
 				return err
 			}
@@ -68,7 +71,7 @@ func (pe *shellVariablesEncoder) doEncode(w *io.Writer, node *CandidateNode, pat
 		for index := 0; index < len(node.Content); index = index + 2 {
 			key := node.Content[index]
 			value := node.Content[index+1]
-			err := pe.doEncode(w, value, appendPath(path, key.Value))
+			err := pe.doEncode(w, value, pe.appendPath(path, key.Value))
 			if err != nil {
 				return err
 			}
@@ -81,7 +84,7 @@ func (pe *shellVariablesEncoder) doEncode(w *io.Writer, node *CandidateNode, pat
 	}
 }
 
-func appendPath(cookedPath string, rawKey interface{}) string {
+func (pe *shellVariablesEncoder) appendPath(cookedPath string, rawKey interface{}) string {
 
 	// Shell variable names must match
 	//    [a-zA-Z_]+[a-zA-Z0-9_]*
@@ -126,7 +129,7 @@ func appendPath(cookedPath string, rawKey interface{}) string {
 		}
 		return key
 	}
-	return cookedPath + "_" + key
+	return cookedPath + pe.prefs.KeySeparator + key
 }
 
 func quoteValue(value string) string {