Implement a new component called `d-multi-select` which allows to select multiple items.

It has the following features:
- accepts a `loadFn` to load data
- yield result to customise results
- yield selection to customise selection

Usage:

```gjs
 <DMultiSelect
  @loadFn={{this.loadData}}
  @onchange={{this.onChange}}
  @selection={{this.selection}}
>
  <:selection as |item|>{{item.name}}</:selection>
  <:result as |result|>{{result.name}}</:result>
</DMultiSelect>

```gjs
<ChatChannelMultiSelect
  @onchange={{fn (mut this.selectedChannels)}}
  @initialIds={{array 2}}
  @selection={{this.selectedChannels}}
>
    <:selection as |item|>{{item.name}}</:selection>
    <:result as |result|>{{result.name}}</:result>
</ChatChannelMultiSelect>