Allow to specify possible actions, subject types and conditions for Ability

[stalniy]

Allow to specify which subjects Ability can use. Affects:

• Ability
  • can
  • cannot
  • update
  • Rule & RawRule
• AbilityBuilder
  • can
  • cannot
• ForbiddenError
• casl/aurelia
• casl/angular
• casl/vue
• casl/react
• casl/mongoose

For example:

import { Ability, AbilityOptions } from '@casl/ability';
import { Post, User } from './models'

type AppSubjects = 'Post' | 'User' | 'all'
// or
type AppSubjects = Post | User | 'all'
interface AppAbilityOptions extends AbilityOptions<AppSubjects> {}

const ability = new Ability<AppAbilityOptions>([]);

// alternatively it can be (need to decide whether it's possible to keep this thing simpler)

const ability = new Ability<Subjects>([]);

// or
const ability = AbilityBuilder.define<Subjects>((can, cannot) => {
  can('read', 'User') // ok
  can('read', 'user') // error
})
// or
const { can, cannot, rules } = AbilityBuilder.extract<Subjects>();

can('read', 'User') // ok
can('read', 'user') // error

// so then
ability.can('read', 'user') // error, but
ability.can('read', 'User') // ok