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/Formatter.cpp

@@ -33,7 +33,7 @@ namespace Shell {
 
 String Formatter::format()
 {
-    auto node = Parser(m_source).parse();
+    auto node = m_root_node ? m_root_node : Parser(m_source).parse();
     if (m_cursor >= 0)
         m_output_cursor = m_cursor;
 
@@ -472,6 +472,27 @@ void Formatter::visit(const AST::IfCond* node)
     visited(node);
 }
 
+void Formatter::visit(const AST::ImmediateExpression* node)
+{
+    will_visit(node);
+    test_and_update_output_cursor(node);
+
+    current_builder().append("${");
+    TemporaryChange<const AST::Node*> parent { m_parent_node, node };
+
+    current_builder().append(node->function_name());
+
+    for (auto& node : node->arguments()) {
+        current_builder().append(' ');
+        node.visit(*this);
+    }
+
+    if (node->has_closing_brace())
+        current_builder().append('}');
+
+    visited(node);
+}
+
 void Formatter::visit(const AST::Join* node)
 {
     will_visit(node);