fix: move svelteHTML to load the correct svelte/element

[jasonlyu123]

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For sveltejs/language-tools#2109. This file is currently in the language-tools repo and ships with svelte2tsx, but the problem is that TypeScript will load the svelte/elements version that comes with the svelte-language-server. Moving this to the Svelte core makes TypeScript always resolve it to the user-installed/workspace version of Svelte. Making it resolve to the user version also fixes the problem with the declaration merging of declare module "svelte/elements". This allows the user/library to add custom types with it. For example:

// app.d.ts
import { HTMLButtonAttributes } from 'svelte/elements'

declare module 'svelte/elements' {
    export interface SvelteHTMLElements {
        'custom-button': HTMLButtonAttributes;
    }

    export interface HTMLButtonAttributes {
        'veryexperimentalattribute': string;
    }
}

This file is meant to be loaded by ts or svelte-language-server. It's not meant to be user-facing. So this doesn't need to be in the package.json export and doesn't need documentation on its usage. @dummdidumm What do you think about the file naming and the declare module thing?

[changeset-bot]

🦋 Changeset detected

Latest commit: 272909b

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

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svelte	Minor

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[dummdidumm]

Name sounds good to me! I'm not sure what you mean by the "declare module thing"? Do you mean that people are now able to extend svelte/elements? Because yes, that's a great side effect of this.

Btw this needs a changeset.

[dummdidumm]

Looks good! Let's try this out locally first by adjusting the language-tools and trying it on a local copy of Svelte.