Use jobleonard improvements

[Artoria2e5]

Over at https://observablehq.com/@jobleonard/mario-klingemans-stackblur there is a modernized shift table. There are a few things useful to this repo:

• Correctness: the shift operator is corrected from >> to >>> so integers stay okay until the intended 255 radius. And lots of | 0 to make sure.
• Simplicity:
  • The shift table is removed in favor of a fixed 24-shift so values become more readable
  • Since stacks only work one way, an array is used instead
  • Fewer stray variables for cleaner code
• Modern JS

[brettz9]

In what way is the JS more modern? The source here is pretty modern.

Before accepting any non-obvious changes, I personally think we need some mocha + nyc testing, using some canvas or image testing library, to ensure current behavior is working as intended.