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/Shell.cpp

@@ -1518,6 +1518,26 @@ Vector<Line::CompletionSuggestion> Shell::complete_option(const String& program_
     return suggestions;
 }
 
+Vector<Line::CompletionSuggestion> Shell::complete_immediate_function_name(const String& name, size_t offset)
+{
+    Vector<Line::CompletionSuggestion> suggestions;
+
+#define __ENUMERATE_SHELL_IMMEDIATE_FUNCTION(fn_name)                            \
+    if (auto name_view = StringView { #fn_name }; name_view.starts_with(name)) { \
+        suggestions.append({ name_view, " " });                                  \
+        suggestions.last().input_offset = offset;                                \
+    }
+
+    ENUMERATE_SHELL_IMMEDIATE_FUNCTIONS();
+
+#undef __ENUMERATE_SHELL_IMMEDIATE_FUNCTION
+
+    if (m_editor)
+        m_editor->suggest(offset);
+
+    return suggestions;
+}
+
 void Shell::bring_cursor_to_beginning_of_a_line() const
 {
     struct winsize ws;