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/Forward.h

@@ -57,6 +57,7 @@ class Glob;
 class HistoryEvent;
 class Execute;
 class IfCond;
+class ImmediateExpression;
 class Join;
 class MatchExpr;
 class Or;
@@ -72,6 +73,7 @@ class Juxtaposition;
 class StringLiteral;
 class StringPartCompose;
 class SyntaxError;
+class SyntheticNode;
 class Tilde;
 class VariableDeclarations;
 class WriteAppendRedirection;