Shell: Add a (very basic) formatter

Shell/Forward.h

@@ -35,5 +35,43 @@ class Value;
 class SyntaxError;
 class Pipeline;
 struct Rewiring;
+class NodeVisitor;
+
+class PathRedirectionNode;
+class And;
+class ListConcatenate;
+class Background;
+class BarewordLiteral;
+class CastToCommand;
+class CastToList;
+class CloseFdRedirection;
+class CommandLiteral;
+class Comment;
+class DynamicEvaluate;
+class DoubleQuotedString;
+class Fd2FdRedirection;
+class FunctionDeclaration;
+class ForLoop;
+class Glob;
+class Execute;
+class IfCond;
+class Join;
+class MatchExpr;
+class Or;
+class Pipe;
+class ReadRedirection;
+class ReadWriteRedirection;
+class Sequence;
+class Subshell;
+class SimpleVariable;
+class SpecialVariable;
+class Juxtaposition;
+class StringLiteral;
+class StringPartCompose;
+class SyntaxError;
+class Tilde;
+class VariableDeclarations;
+class WriteAppendRedirection;
+class WriteRedirection;
 
 }