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/CMakeLists.txt

@@ -1,20 +1,21 @@
 set(SOURCES
-    AST.cpp
-    Builtin.cpp
-    Formatter.cpp
-    Job.cpp
-    NodeVisitor.cpp
-    Parser.cpp
-    Shell.cpp
-    SyntaxHighlighter.cpp
-)
+        AST.cpp
+        Builtin.cpp
+        Formatter.cpp
+        ImmediateFunctions.cpp
+        Job.cpp
+        NodeVisitor.cpp
+        Parser.cpp
+        Shell.cpp
+        SyntaxHighlighter.cpp
+        )
 
 serenity_lib(LibShell shell)
-target_link_libraries(LibShell LibCore LibLine LibSyntax)
+target_link_libraries(LibShell LibCore LibLine LibSyntax LibRegex)
 
 set(SOURCES
-    main.cpp
-)
+        main.cpp
+        )
 
 serenity_bin(Shell)
 target_link_libraries(Shell LibShell)
@@ -22,5 +23,5 @@ target_link_libraries(Shell LibShell)
 install(DIRECTORY Tests/ DESTINATION usr/Tests/Shell
         PATTERN "Tests/*"
         PERMISSIONS OWNER_EXECUTE OWNER_WRITE OWNER_READ
-                   GROUP_EXECUTE GROUP_READ
-                   WORLD_EXECUTE WORLD_READ)
+        GROUP_EXECUTE GROUP_READ
+        WORLD_EXECUTE WORLD_READ)