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/Parser.h

@@ -98,6 +98,7 @@ private:
     RefPtr<AST::Node> parse_glob();
     RefPtr<AST::Node> parse_brace_expansion();
     RefPtr<AST::Node> parse_brace_expansion_spec();
+    RefPtr<AST::Node> parse_immediate_expression();
 
     template<typename A, typename... Args>
     NonnullRefPtr<A> create(Args... args);
@@ -238,6 +239,7 @@ list_expression :: ' '* expression (' '+ list_expression)?
 expression :: evaluate expression?
             | string_composite expression?
             | comment expression?
+            | immediate_expression expression?
             | history_designator expression?
             | '(' list_expression ')' expression?
 
@@ -268,6 +270,10 @@ variable :: '$' identifier
 
 comment :: '#' [^\n]*
 
+immediate_expression :: '$' '{' immediate_function expression* '}'
+
+immediate_function :: identifier       { predetermined list of names, see Shell.h:ENUMERATE_SHELL_IMMEDIATE_FUNCTIONS }
+
 history_designator :: '!' event_selector (':' word_selector_composite)?
 
 event_selector :: '!'                  {== '-0'}