Introduce Await component for nicer template usage

[chriskrycho]

The existing load function/helper combo gets the job done, but this new component has a similar top-level API but requires no if-based branching. Instead, it uses named blocks to render the resulting states.

import { Async } from 'ember-async-data';

<template>
  <Async @data={{Promise.resolve 123}}>
    <:pending>Still waiting...</:pending>
    <:resolved as |value|>
      The value is: {{value}}.
    </:resolved>
    <:rejected>Whoops! Something went wrong!</:rejected>
  </Async>
</template>

This ends up being both briefer and less error prone, not least since it provides the resolved value automatically from the resolved block, whereas expressly checking {{#if someAsyncData.isResolved}} allowed the further {{someAsyncData.value}} access but did not provide it automatically.

[chriskrycho]

This seemed to be the best approach we had when I spiked this out a few months ago, but I'm totally up for a different approach if we have something better now.

[NullVoxPopuli]

the v2 addon blueprint, via @embroider/addon-dev does this for you now.
However, pending:

• Glint does not support gts imports with the extension embroider-build/addon-blueprint#194
• gts files are left with side-effecting imports embroider-build/addon-blueprint#195

I use this for now:
https://github.com/universal-ember/ember-primitives/blob/main/ember-primitives/rollup.config.mjs#L20

[NullVoxPopuli]

Thanks for adding more gjs/gts files!!