(feature): Add support for default values and validation rules

[amckinney]

This adds support for default values and validation rules for integer, double, and string types.

The Fern definition looks like the following, where each of the relevant primitive types has a full example:

types:
  Type:
    docs: |
      Defines properties with default values and validation rules.
    properties:
      decimal:
        type: double
        default: 1.1
        validation:
          exclusiveMin: true
          exclusiveMax: false
          min: 1.1
          max: 2.2
          multipleOf: 1.1
      even:
        type: integer
        default: 42
        validation:
          exclusiveMin: false
          exclusiveMax: true
          min: 0
          max: 100
          multipleOf: 2
      name:
        type: string
        default: fern
        validation:
          minLength: 3
          maxLength: 10
          pattern: ^[a-z]+$