Add @contentHtmlTag

[mkszepp]

Similar to @triggerHtmlTag (#1033), we need to introduce @contentHtmlTag to achieve full Glint compliance in next major.
This allows addon consumers to adopt the new approach in v8 and avoid having to make this change during the migration to v9.

Before:

<BasicDropdown as |dd|>
  <dd.Trigger>Click me!</dd.Trigger>
  <dd.Content @htmlTag="span">
    Content
  </dd.Content>
</BasicDropdown>

After:

<BasicDropdown @contentHtmlTag="span" as |dd|>
  <dd.Trigger>Click me!</dd.Trigger>
  <dd.Content>
    Content
  </dd.Content>
</BasicDropdown>