Limit Amount of a Widget Type in an Area

I know I can limit the number of widgets in an area with

```
main: {
type: 'area',
options: {
widgets: {
'@apostrophecms/rich-text': {},
'@apostrophecms/image': {},
'@apostrophecms/video': {}
},
max: 3
}
}
```

but is there any way to limit the amount of any specific widget in that area? e.g.

```
main: {
type: 'area',
options: {
widgets: {
'@apostrophecms/rich-text': {},
'@apostrophecms/image': {},
'@apostrophecms/video': {
max: 1 // 👈 this puppy here means only one video widget in this area
}
},
max: 3
}
}
```

I know I could make another area, but the order of areas in the page is still fixed; this feature would allow users to re-organise the area content avoiding this fixed-layout problem.

Start with the area configuration and existing area-wide max behavior shown in the issue, then locate the implementation and tests that handle those options. Done means a per-widget maximum can be configured without preventing users from reordering mixed widgets, with the behavior covered by a regression test.

Written by the indexing model from the issue text.