levenshteinDistance doesn't correctly handle code points over U+FFFF

[lionel-rowe]

Describe the bug

levenshteinDistance doesn't correctly handle code points over U+FFFF.

Steps to Reproduce

Deno.test("levenshteinDistance() handles code points above U+FFFF", () => {
  assertEquals(levenshteinDistance("💩", "x"), 1);
  assertEquals(levenshteinDistance("x", "💩"), 1);
  // first surrogate same
  assertEquals(levenshteinDistance("💩", "💫"), 1);
  // both surrogates different
  assertEquals(levenshteinDistance("💩", "🦄"), 1);
});

Currently, of the above tests, only "💩", "💫" passes.

Expected behavior

All four cases pass.

Environment

• OS: Ubuntu 20.04 on WSL
• deno version: deno 1.46.3
• std version: @std/text@1.0.6