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/immediate.sh

@@ -0,0 +1,85 @@
+#!/bin/sh
+
+source $(dirname "$0")/test-commons.inc
+
+# Length
+
+if test "${length foo}" -ne 3 {
+  fail invalid length for bareword
+}
+
+if test "${length "foo"}" -ne 3 {
+  fail invalid length for literal string
+}
+
+if test "${length foobar}" -ne 6 {
+  fail length string always returns 3...\?
+}
+if test "${length string foo}" -ne 3 {
+  fail invalid length for bareword with explicit string mode
+}
+
+if test "${length list foo}" -ne 1 {
+  fail invalid length for bareword with explicit list mode
+}
+
+if test "${length (1 2 3 4)}" -ne 4 {
+  fail invalid length for list
+}
+
+if test "${length list (1 2 3 4)}" -ne 4 {
+  fail invalid length for list with explicit list mode
+}
+
+if test "${length_across (1 2 3 4)}" != "1 1 1 1" {
+  fail invalid length_across for list
+}
+
+if test "${length_across list ((1 2 3) (4 5))}" != "3 2" {
+  fail invalid length_across for list with explicit list mode
+}
+
+if test "${length_across string (foo test)}" != "3 4" {
+  fail invalid length_across for list of strings
+}
+
+# remove_suffix and remove_prefix
+if test "${remove_suffix .txt foo.txt}" != "foo" {
+  fail remove_suffix did not remove suffix from a single entry
+}
+
+if test "${remove_suffix .txt (foo.txt bar.txt)}" != "foo bar" {
+  fail remove_suffix did not remove suffix from a list
+}
+
+if test "${remove_prefix fo foo.txt}" != "o.txt" {
+  fail remove_prefix did not remove prefix from a single entry
+}
+
+if test "${remove_prefix x (xfoo.txt xbar.txt)}" != "foo.txt bar.txt" {
+  fail remove_prefix did not remove prefix from a list
+}
+
+# regex_replace
+if test "${regex_replace a e wall}" != "well" {
+  fail regex_replace did not replace single character
+}
+
+if test "${regex_replace a e waaaall}" != "weeeell" {
+  fail regex_replace did not replace multiple characters
+}
+
+if test "${regex_replace '(.)l' 'e\1' hall}" != "heal" {
+  fail regex_replace did not replace with pattern
+}
+
+# split
+
+if test "${split 'x' "fooxbarxbaz"}" != "foo bar baz" {
+  fail split could not split correctly
+}
+
+if test "${split '' "abc"}" != "a b c" {
+  fail split count not split to all characters
+}
+pass