Upgrade svelte to 3.50.0

[metonym]

Currently, the Svelte compiler is unable to recognize svelte:element tags or @const tags in #if blocks. This PR upgrades svelte to the latest minor version.

In the AST Explorer, the Svelte compiler is unable to parse the following code. It works in the Svelte REPL when using the latest minor version.

<!-- 3.48.0 -->
{#if true}
  {@const a = 44}
    {a}
{/if}

<!-- 3.47.0 -->
<svelte:element this="h6">
  h6 tag
</svelte:element>