Add string.split() to the canonical spec (#3502)

See #1950

spec/built-in-modules/string.md

@@ -10,6 +10,7 @@ This built-in module is available from the URL `sass:string`.
   * [`length()`](#length)
   * [`quote()`](#quote)
   * [`slice()`](#slice)
+  * [`split()`](#split)
   * [`to-lower-case()`](#to-lower-case)
   * [`to-upper-case()`](#to-upper-case)
   * [`unique-id()`](#unique-id)
@@ -58,6 +59,72 @@ slice($string, $start-at, $end-at: -1)
 
 This function is also available as a global function named `str-slice()`.
 
+### `split()`
+
+```
+split($string, $separator, $limit: null)
+```
+
+* If `$string` is not a string, throw an error.
+
+* If `$separator` is not a string, throw an error.
+
+* If `$limit` is a value other than an integer or `null`, throw an error.
+
+* If `$limit` is less than 1, throw an error.
+
+* If `$string` is an empty string, return a list with `$string` as the only 
+  item.
+
+* Let `split-list` be an empty list.
+
+* If `$limit` is `null`, set `$limit` to the value of calling 
+  `string.length($string)`.
+
+* Let `split-counter` equal 0.
+
+* While `split-counter <= $limit` and `string.length($string) > 0`:
+
+  * If `split-counter == $limit`:
+
+    * Append `$string` to `split-list`.
+
+    * Set `$string` to an empty string. 
+
+  * Otherwise:
+
+    * If `$separator` is an empty string:
+
+      * Let `code-point` be the value of calling `string.slice($string, 1, 1)`.
+
+      * Append `code-point` to `split-list`.
+
+      * Set `$string` to `string.slice($string, 2)`.
+
+      * Increase `split-counter` by 1.
+
+    * Otherwise:
+
+      * Let `index` be the result of calling 
+        `string.index($string, $separator)`.
+
+      * If `index` is `null`, append `$string` to `split-list` and set `$string` 
+        to an empty string.
+
+      * Otherwise:
+
+        * Let `current-substring` be the result of calling
+          `string.slice($string, 1, index - 1)`.
+
+        * Append `current-substring` to `split-list`.
+
+        * Set `$string` to 
+          `string.slice($string, index + string.length($separator))`.
+
+        * Increase `split-counter` by 1.
+
+* Return `split-list` as a bracketed, comma-separated list.
+
 ### `to-lower-case()`
 
 ```