Date comparison isn't very useful

Nobody has claimed this yet.

Comparing dates with either `.equal` or `.is` results in some very obtuse output which makes it impossible to debug.

# input

```js
const { test } = require('uvu')
const assert = require('uvu/assert')

test('dates', () => {
const a = new Date()
const b = new Date(new Date().setMonth(5))
assert.is(a, b)
})

test.run()
```
# output

```sh
npx uvu
index.spec.js
✘ (0 / 1)

FAIL "dates"
Expected values to be strictly equal: (is)

++"2021-06-04T11:40:40.529Z" (Expected)
-- {} (Actual)
^^^^^^^^^^^^^^^^^^^^^^^^^^
at assert (/home/ant/Projects/uvutest/node_modules/uvu/assert/index.js:31:8)
at Object.is (/home/ant/Projects/uvutest/node_modules/uvu/assert/index.js:39:2)
at Object.handler (/home/ant/Projects/uvutest/index.spec.js:7:12)
at Number.runner (/home/ant/Projects/uvutest/node_modules/uvu/dist/index.js:77:16)
at exec (file:///home/ant/Projects/uvutest/node_modules/uvu/dist/index.mjs:131:39)
at Module.run (file:///home/ant/Projects/uvutest/node_modules/uvu/run/index.mjs:13:8)
at async /home/ant/Projects/uvutest/node_modules/uvu/bin.js:26:5

Total: 1
Passed: 0
Skipped: 0
Duration: 2.36ms
```

Reproduce the case in index.spec.js and start at uvu/assert/index.js, where the reported Object.is output is shown in the stack trace. Compare the failure output for Date values with the expected behavior described in the issue, then add or update assertion coverage so date comparisons produce useful diagnostics.

Written by the indexing model from the issue text.