Add @parcel/macros package with macro context TS definitions

[devongovett]

For use in macros that want to create assets or change caching behavior. The this context can be typed like this:

import {MacroContext} from '@parcel/macros';

export function myMacro(this: MacroContext) {
  // ...
}

In the future we could potentially put more than just types in this package, e.g. if we wanted to have some built-in macros.