fix: replace deprecated String.prototype.substr() (#155)

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated

Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

lib/ini.js

@@ -182,9 +182,8 @@ const unsafe = (val, doUnesc) => {
   if (isQuoted(val)) {
     // remove the single quotes before calling JSON.parse
     if (val.charAt(0) === "'") {
-      val = val.substr(1, val.length - 2)
+      val = val.slice(1, -1)
     }
-
     try {
       val = JSON.parse(val)
     } catch (_) {}