Bump core from 9f15ba5 to 216d28e (#25)

Bumps [core](https://github.com/microsoft/typespec) from `9f15ba5` to
`216d28e`.
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/microsoft/typespec/commit/216d28ebe4e6ae015068e1de99615f032aec7c5b"><code>216d28e</code></a>
VSCode extension: Use rollup for dev build too (<a
href="https://redirect.github.com/microsoft/typespec/issues/2730">#2730</a>)</li>
<li>See full diff in <a
href="https://github.com/microsoft/typespec/compare/9f15ba550211b3ea81828753955d5faa608ab79c...216d28ebe4e6ae015068e1de99615f032aec7c5b">compare
view</a></li>
</ul>
</details>
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