fix: replace deprecated String.prototype.substr() (#38)

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

Signed-off-by: Tobias Speicher <[email protected]>

lib/index.js

@@ -37,7 +37,7 @@ function abbrev (...args) {
       abbrevs[current] = current
       continue
     }
-    for (let a = current.substr(0, j); j <= cl; j++) {
+    for (let a = current.slice(0, j); j <= cl; j++) {
       abbrevs[a] = current
       a += current.charAt(j)
     }