refactor!: remove pcre2el package

BREAKING CHANGE: This removes the pcre2el package, which Doom was using
solely for one function to escape PCREs. In the interest of thinning out
Doom's core, I've hoisted a simpler version of the function into Doom's
stdlib so I can remove the dependency.

lisp/lib/strings.el

@@ -0,0 +1,15 @@
+;;; lisp/lib/strings.el -*- lexical-binding: t; -*-
+
+;;;###autoload
+(defun doom-pcre-quote (str)
+  "Like `reqexp-quote', but for PCREs."
+  (let ((special '(?. ?^ ?$ ?* ?+ ?? ?{ ?\\ ?\[ ?\| ?\())
+        (quoted nil))
+    (mapc (lambda (c)
+            (when (memq c special)
+              (push ?\\ quoted))
+            (push c quoted))
+          str)
+    (concat (nreverse quoted))))
+
+;;; end of strings.el