Replace String.prototype.substr() occurrences with String.prototype.substring()

As outlined in https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr, which refers to the ECMA-262 specification, using the `substr` function is advised against.

Hence this PR, which replaces all remaining `substr` occurrences with `substring` instead. Please refer to https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr#Syntax respectively https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substring#Syntax for the differences between the two functions.

Note that in most cases in the code-base there's only one argument passed to `substr`, and those require no other changes except replacing "substr" with "substring". For the other cases, the `substr(start, length)` calls are changed to `substring(start, start + length)` instead.

src/core/cmap.js

@@ -228,7 +228,7 @@ class CMap {
     while (low <= high) {
       this._map[low++] = dstLow;
       // Only the last byte has to be incremented.
-      dstLow = dstLow.substr(0, lastByte) +
+      dstLow = dstLow.substring(0, lastByte) +
                String.fromCharCode(dstLow.charCodeAt(lastByte) + 1);
     }
   }