[v6] Implement checkboxes for booleans

[josdejong]

For values which are boolean, Implement checkboxes left from the value to easily toggle true/false like in the following image:

Rendering a checkbox is not the only special rendering of a value. For example an enum of a JSONSchema will be rendered as a drop down (which replaced the free input field). In the end it should become completely customizable how a value is rendered. This should be kept in mind when implementing this feature.

Related: #391

[dougn7]

Hello. Theres a way to disable this? I'd like to implement a verification myself.

[josdejong]

@dougn7 it shouldn't be to hard to hide the checkboxes with some CSS if you don't want them.

[dougn7]

It works! Thanks. Thank guys for the hard work. I could help implement some stuff, but im not experienced enough with Javascript best practices and Git..

[josdejong]

Thanks for your kind words 👍