Historically, the legacy importer shim has strongly assumed that all
calls to `canonicalize()` would doubled: once as a URL resolved
relative to the current importer, and again as a URL passed to the
importers list. However, this relies on buggy, non-spec-compliant
behavior in Dart Sass: absolute URLs should never be passed to the
current importer, only to the global list of importers.

This change avoids relying on that behavior by instead disambiguating
relative loads using a custom URL protocol prefix. All canonical URLs
passed to Dart Sass now have the prefix `legacy-importer-` added to
the beginning. This allows us to disambiguate relative loads, and
ignore them for non-`file:` URLs, since relative loads and _only_
relative loads will have schemes beginning with `legacy-importer-`.

To avoid regressing the developer experience, we then strip these
prefixes from all URLs before they're passed to any developer-facing
APIs or displayed to the user.

We need to copy the entrypoing typings file, as described in [the
TypeScript docs]:

> It’s important to note that the CommonJS entrypoint and the ES
> module entrypoint each needs its own declaration file, even if the
> contents are the same between them. Every declaration file is
> interpreted either as a CommonJS module or as an ES module, based on
> its file extension and the `"type"` field of the `package.json`, and
> this detected module kind must match the module kind that Node will
> detect for the corresponding JavaScript file for type checking to be
> correct. Attempting to use a single `.d.ts` file to type both an ES
> module entrypoint and a CommonJS entrypoint will cause TypeScript to
> think only one of those entrypoints exists, causing compiler errors
> for users of the package.

[the TypeScript docs]: https://www.typescriptlang.org/docs/handbook/esm-node.html

Closes #247

Closes #3247

Bumps [minipass](https://github.com/isaacs/minipass) from 7.0.3 to 7.0.4.
- [Changelog](https://github.com/isaacs/minipass/blob/main/CHANGELOG.md)
- [Commits](isaacs/minipass@v7.0.3...v7.0.4)

---
updated-dependencies:
- dependency-name: minipass
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Co-authored-by: Natalie Weizenbaum <nweiz@google.com>

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