Shell: Add support for 'immediate' expressions as variable substitutions

This commit adds a few basic variable substitution operations:
- length
    Find the length of a string or a list
- length_across
    Find the lengths of things inside a list
- remove_{suffix,prefix}
    Remove a suffix or a prefix from all the passed values
- regex_replace
    Replace all matches of a given regex with a given template
- split
    Split the given string with the given delimiter (or to its
    code points if the delimiter is empty)
- concat_lists
    concatenates any given lists into one

Closes #4316 (the ancient version of this same feature)

Userland/Shell/NodeVisitor.cpp

@@ -139,6 +139,12 @@ void NodeVisitor::visit(const AST::IfCond* node)
         node->false_branch()->visit(*this);
 }
 
+void NodeVisitor::visit(const AST::ImmediateExpression* node)
+{
+    for (auto& node : node->arguments())
+        node.visit(*this);
+}
+
 void NodeVisitor::visit(const AST::Join* node)
 {
     node->left()->visit(*this);
@@ -224,6 +230,10 @@ void NodeVisitor::visit(const AST::SyntaxError*)
 {
 }
 
+void NodeVisitor::visit(const AST::SyntheticNode*)
+{
+}
+
 void NodeVisitor::visit(const AST::Tilde*)
 {
 }