[v.0.4] Add default initializer via $default method

[soulofmischief]

This PR adds a symbol that can be used to key a default initializer function in a component. It throws if the initializer isn't a function.

The API is such:

import { $default, Default, addComponent, addEntity, createWorld } from 'bitECS'

const world = createWorld()
const eid = addEntity( world )

const MyComponent = {
  x: [] as number[],
  y: [] as number[],
  [$default]( eid, { x = 1, y = 1 } = {}) {
    MyComponent.x[ eid ] = x
    MyComponent.y[ eid ] = y
  }
}

addComponent( world, eid, MyComponent )
assert( MyComponent.x[ eid ] === 1 )

// or:
addComponent( world, eid, Default( MyComponent, { x: 42 }))

Not sure if you wanted direct support for a default initializer, but this seems like a non-intrusive way to do it. Anything I should change/add if you want to merge?

I know you mentioned in #105 that you wanted to avoid invoking GC or creating unnecessary objects, but this approach is entirely opt-in, doesn't modify the API, and only introduces one conditional check for the $default property.

It's up to the user to avoid invoking GC with the default method, but it's no different than the user just setting these values manually every time after adding a component, or using a prefab. We could also include a stronger warning in the docs. The Default() wrapper also gives the user a way to avoid double initialization.

[soulofmischief]

Should $default and $defaultParams be non-enumerable?