Remove use of string-lines (#4217) (#4219)

* Remove use of `string-lines` (#4217)

The Elisp function `string-lines` is not available prior to Emacs
version 28.1, so its presence breaks compatibility with earlier
versions.

* Use `s-lines` to split lines

lsp-completion.el

@@ -355,7 +355,7 @@ The MARKERS and PREFIX value will be attached to each candidate."
          ;; `lsp-completion-start-point' above might be from cached/previous completion and
          ;; pointing to a very distant point, which results in `prefix' being way too long.
          ;; So let's consider only the first line.
-         (prefix (car (string-lines prefix)))
+         (prefix (car (s-lines prefix)))
          (prefix-len (length prefix))
          (prefix-pos 0)
          (label (downcase candidate))