Add dedent to core.

Makes longstrings easier to use - can be combined with comptime
for overhead free long strings.

src/boot/boot.janet

@@ -1332,6 +1332,27 @@
   (if (not= i len) (array/push ret (slicer ind i)))
   ret)
 
+(defn dedent
+  "Remove indentation after concatenating the arguments. Works by removing
+  leading whitespace, and then removing that same pattern of whitepsace after
+  new lines."
+  [& xs]
+  (def x (string ;xs))
+  (def first-letter (find-index (fn [c] (and (not= c (chr "\n"))
+                                             (not= c (chr " "))
+                                             (not= c (chr "\t"))
+                                             (not= c (chr "\r")))) x))
+  (if (not first-letter) (break ""))
+  (def leading-whitespace (string/slice x 0 first-letter))
+  (def indent (last (string/split "\n" leading-whitespace)))
+  (if (and indent (not= indent ""))
+    (let [y (string/replace-all (string "\n" indent) "\n" (string/replace indent "" x))]
+      # Remove trailing newline to mimic long string newline omission.
+      (if (= (chr "\n") (last y))
+        (slice y 0 -2)
+        y))
+    x))
+
 ###
 ###
 ### IO Helpers