Bump core from 224e725 to 393f40a (#1079)

Bumps [core](https://github.com/microsoft/typespec) from `224e725` to
`393f40a`.
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/microsoft/typespec/commit/393f40a4da397d5c1fec97948cc3a0e0dbb04e00"><code>393f40a</code></a>
upgrade TypeSpec to 0.57.0 and support Nullable feature (<a
href="https://github.com/microsoft/typespec/issues/3654">#3654</a>)</li>
<li><a
href="https://github.com/microsoft/typespec/commit/17c6817430e8f430390387e350a9d52c8c9b5824"><code>17c6817</code></a>
Remove unused macos reference (<a
href="https://github.com/microsoft/typespec/issues/3604">#3604</a>)</li>
<li><a
href="https://github.com/microsoft/typespec/commit/01626ea1f4cb6d8ea21bef9c524b99aabfc8e1cc"><code>01626ea</code></a>
Delete ApiTypes (<a
href="https://github.com/microsoft/typespec/issues/3650">#3650</a>)</li>
<li><a
href="https://github.com/microsoft/typespec/commit/b61de10ed87c1d2c98e722bb14e1186839f6e05f"><code>b61de10</code></a>
Add support to generate a model as struct (<a
href="https://github.com/microsoft/typespec/issues/3635">#3635</a>)</li>
<li><a
href="https://github.com/microsoft/typespec/commit/9d48ad1a56bb398283f94093a005d58d3d27f51f"><code>9d48ad1</code></a>
use test file assets vs string builder (<a
href="https://github.com/microsoft/typespec/issues/3640">#3640</a>)</li>
<li>See full diff in <a
href="https://github.com/microsoft/typespec/compare/224e725e43e51b8cc427779377fa4e351a25a38d...393f40a4da397d5c1fec97948cc3a0e0dbb04e00">compare
view</a></li>
</ul>
</details>
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Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Mark Cowlishaw <[email protected]>