Provide a way of defining schemas for arrays as root nodes

As an example
```ruby
Dry::Schema.JSON do
each do
schema do
required(:name).filled(:str?)
required(:age).filled(:int?)
end
end
end
```

Read the existing Dry::Schema.JSON entry point and the each/schema DSL behavior first, then trace how schemas currently handle object roots. Define how the shown array-root example should be accepted and validated, and confirm completion with coverage for array roots and nested name/age rules.

Written by the indexing model from the issue text.