Update core components to support multiple=true on checkbox inputs

[andrewtimberlake]

Currently, if you want to use the core components for multi value checkboxes, you can’t.
This change maintains backwards compatibility with standard on/off checkboxes and adds the ability to do this:

<form>
  <.input type="checkbox" name="field[active][]" value="true" label="Yes" multiple={true} />
  <.input type="checkbox" name="field[active][]" value="false" label="No" multiple={true} />
</form>

[SteffenDE]

What is a "multi-value checkbox"? A checkbox is either on or off (or maybe indeterminate) and even in your example, how does it make sense to have an active field that can be both yes and no at the same time?

CoreComponents are meant to be modified if your app needs it. To me, this looks like such a thing that may be needed in some rare circumstances, but not worth being included by default.

[andrewtimberlake]

That’s a simple example, but a practical one (granted this could more logically be radio buttons).

A multi-value checkbox is a collection of checkboxes that produce an array of results. It is similar to using a multi value select but sometimes it’s a much nicer UI.

I have made this change in a number of different projects:

• check off from a list of possible files to attach to a message
• check off from a list of fields to include when sending a webhook
• check off all the regions this business belongs to
• check off all the categories this project belongs to

[andrewtimberlake]

And I’m about to implement another: checking off rows of a table (edit multiple, delete multiple etc).
Similar to this:

[SteffenDE]

Those examples make more sense :)

I still think core components should focus on the basic functionality. Since the generators don't use this, I'm not sure that we should add it. Other opinions? @josevalim @chrismccord