feat(nest): metadata decorator

[CarsonF]

This abstracts the reflect-metadata library to provide strictly typed decorators with an easier to use API.

Simple marker tag

const Tag = createMetadataDecorator();

@Tag()
class Foo {}

Tag.get(Foo) // => true | undefined
// === true

Add data to be stored

const Tag = createMetadataDecorator({
  // This function declares the args that are exposed to the decorator
  // And the return type declares the stored value shape
  setter: (name: string, opts?: { age?: number }) => ({
    name,
    age: opts?.age,
  })
});

@Tag('Bob', { age: 50 })
class Foo {}

Tag.get(Foo) // => { name: string, age?: number } | undefined
// === { name: 'Bob', age: 50 }

Labels with merging

const Label = createMetadataDecorator({
  type: ['class'] // only allow for classes
  setter: (label: string) => [label],
  merge: ({ next, previous }) => [...previous ?? [], ...next],
});

@Label('A')
@Label('B')
class Foo {}

Label.get(Foo) // => string[] | undefined
// === ['B', 'A']

[bryanjnelson]

Those tests... 🤤