Re-factor how the ESLint no-var rule is enabled in the src/ folder

This simplifies/consolidates the ESLint configuration slightly in the `src/` folder, and prevents the addition of any new files where `var` is being used.[1]
Hence we no longer need to manually add `/* eslint no-var: error */` in files, which is easy to forget, and can instead disable the rule in the `src/core/` files where `var` is still in use.

---
[1] Obviously the `no-var` rule can, in the same way as every other rule, be disabled on a case-by-case basis where actually necessary.

src/core/murmurhash3.js

@@ -16,7 +16,6 @@
  * Based on https://code.google.com/p/smhasher/wiki/MurmurHash3.
  * Hashes roughly 100 KB per millisecond on i7 3.4 GHz.
  */
-/* eslint no-var: error */
 
 import { isArrayBuffer, isString } from "../shared/util.js";