Add `$().data` for handling `data-*`

I coded up [a mockup of the idea here](https://codepen.io/gingerchew/pen/dyrvYNG/b28d9fd99049ab7b9b93de8c07405724). While there is overlap with `.attr()` I think there could still be a place for it. especially when handling data that can't be coerced to a string.

```js
$('div').attr('data-user', {
fname: 'argyle',
lname: 'ink'
});

$('div').attr('user') // "[object Object]"

// data-user="{"fname":"ginger","lname":"chew"}"
$('div').data('user', {
fname: 'ginger',
lname: 'chew'
});

$('div').data('user') // { fname: 'ginger', lname: 'chew' }

// Needing to set `data-` like this on an object is frustrating
$('div').attr({
'data-fname': 'argyle',
'data-lname': 'ink'
'data-user-id': 1
});

// the `data-` is assumed and don't need to wrap the keys in quotes
// and converting camelCase to kebab-case is handled automatically
$('div').data({
fname: 'ginger',
lname: 'chew',
userId: 2
})
```

The mockup handles set/get/delete, so accepting an object as the first argument like in `.attr()` isn't there yet.

Start by comparing the existing .attr() entry point with the linked CodePen mockup, focusing on the proposed data-* behavior. Clarify the intended set, get, and delete API, including object values, automatic data- prefixing, and camelCase conversion. Done means the agreed behavior is implemented and covered by the project’s tests.

Written by the indexing model from the issue text.