[feat] add SveltePreprocessor utility type

[metonym]

This PR adds a utility to type Svelte preprocessors. It aims to improve the DX for authors writing Svelte preprocessors using TypeScript. It was inspired by #7121, which added a utility Action type to strongly type Svelte actions.

Current DX

The current workflow is to use the PreprocessorGroup type.

import type { PreprocessorGroup } from "svelte/types/compiler/preprocess";

function preprocessor(): Pick<PreprocessorGroup<"markup">> {
  return {
    markup({ content }) {
      return { code: content };
    },
  };
}

However, since a preprocessor is usually a function that returns a function to allow user-provided options, it would be more concise for a utility type to select from the various groups (markup/style/script) and optionally add a generic options as the second parameter.

Proposed DX

The following examples illustrate basic use cases:

• preprocessor with a single group
• preprocessor with multiple groups
• preprocessor with user-provided options

import type { SveltePreprocessor } from "svelte/types/compiler/preprocess";

// Markup-only preprocessor without options
const preprocessMarkup: SveltePreprocessor<"markup"> = () => {
  return {
    markup({ content }) {
      return { code: content };
    },
  };
};

// Markup and style preprocessor without options
const preprocessMarkupStyles: SveltePreprocessor<"markup" | "style"> = () => {
  return {
    markup({ content }) {
      return { code: content };
    },
    style({ content }) {
      return { code: content };
    },
  };
};

// Script preprocessor with user options
// Options are passed as the second parameter
const preprocessWithOptions: SveltePreprocessor<"script", { sourcemap?: boolean }> = (options) => {
  const sourcemap = options?.sourcemap === true;

  return {
    script({ content }) {
      return { code: content, map: sourcemap ? "..." : undefined };
    },
  };
};

[dummdidumm]

Nice!

[Conduitry]

This has been released in 3.50.0. Thank you!