coverage: add `--threshold=<percentage>` option to `deno coverage`

[kt3k]

Follow up of #21325

It'd be good to have --threshold=<percentage> option in deno coverage (and also --coverage-threshold=<percentage> in deno test --coverage).

If the option specified, the process will fail when the coverage number is below the given threshold.

Note:

• jest, nyc, and c8 have configurations for this behavior.
  • These tools allow users to configure them based on 4 different coverage numbers: line coverage, function coverage, branch coverage, and statement coverage. (I think this granularity is overkill. It'd be enough to only allow setting this for line coverage, which seems most commonly used)