docs: Fix link to custom importer

[eliot-akira]

This pull request fixes the link to the custom importer option in the section "Dart Sass in the Browser" in the README.

The original link at /interfaces/StringOptionsWithImporter#importer goes to 404 page not found. In addition, the referenced interface StringOptionsWithImporter is deprecated and links to StringOptions, where the importer option is documented.

The updated URL is: https://sass-lang.com/documentation/js-api/interfaces/stringoptions/#importer

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After creating this PR, I read the contribution guide and learned that I would need to sign the Google Individual CLA. Unfortunately I'd prefer not to create a Google account in order to sign the CLA, or change my Git author email. I hereby declare this PR to be in the public domain. Maybe someone who has signed a CLA can take over and get it merged - otherwise please feel free to close it.

https://opensource.google/documentation/reference/cla

Contributors must sign in with a Google account to sign the CLA or manage their CLA agreements.

One of the most common problems is that the git author email in the commit is not an email address associated with a CLA. The solution is to change the git author email to be an address covered by the CLA. That email should also be added to their GitHub account; it doesn't need to be the primary email, but it should be on the account.

[eliot-akira]

Thank you. I love the Sass project, may it live long and prosper.