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/shellvariables.go

@@ -0,0 +1,14 @@
+package yqlib
+
+type ShellVariablesPreferences struct {
+	KeySeparator string
+}
+
+func NewDefaultShellVariablesPreferences() ShellVariablesPreferences {
+	return ShellVariablesPreferences{
+		KeySeparator: "_",
+	}
+}
+
+var ConfiguredShellVariablesPreferences = NewDefaultShellVariablesPreferences()
+