feat: allow generics on snippets

[paoloricciuti]

Closes #15883

This adds support for the parser for the generic syntax on snippets...this can be useful for stuff like

{#snippet list<TItem>(list: TItem[], show: (item: TItem)=>boolean)}
    {#each list as item}
        {#if show(item)}
             {item}
        {/if}
    {/each}
{/snippet}

to be actually functional needs a respective pr in language-tools

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[changeset-bot]

🦋 Changeset detected

Latest commit: 48d78ab

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Minor

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15915-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15915

[Rich-Harris]

I think we can reuse some existing bracket matching logic (which is already slightly duplicative) — will push a change

[paoloricciuti]

Uh great idea, thanks

[WarningImHack3r]

Oh, that was fast and unexpected, thanks Paolo & Rich!!