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/Tests/test-commons.inc

@@ -4,3 +4,8 @@ fail() {
     echo "FA""IL:" $*
     exit 1
 }
+
+pass() {
+    echo "PA""SS"
+    exit 0
+}