[V2][Docs] Add comments to parcel/packages/core/types/index.js

[brillout]

The idea here is to comment the typings file as a temporary doc. For example:

  // parcel/packages/core/types/index.js
  // ...
  export interface TransformerResult {
+   // `type` is the kind of asset ('js', 'css', etc.).
    type: string;
+   // One of the following `source`, `content`, or `ast` needs to be defined.
+   // The source code of the asset.
    code?: string;
    map?: ?SourceMap;
    content?: Blob;
    ast?: ?AST;
    dependencies?: $ReadOnlyArray<DependencyOptions>;
    includedFiles?: $ReadOnlyArray<File>;
+   // Whether the asset is ....
    isIsolated?: boolean;
+   // I'm explaining what `isInline` is..
    isInline?: boolean;
+   // Same here...
    isSource?: boolean;
    env?: EnvironmentOpts;
    meta?: Meta;
    pipeline?: ?string;
    symbols?: Map<Symbol, Symbol>;
    sideEffects?: boolean;
    uniqueKey?: ?string;
  }
  // ...

Such comments would dramatically help the usage of Parcel's Plugin API.

I would even say that I wouldn't need any additional docs beyond such comments. But my point is to add comments as a temporary quick-n-dirty way to help us understand things before a full-blown doc is written.