Add SwiftPM Add to App Docs

[vashworth]

Description of what this PR is changing or adding, and why:

This PR adds documentation on how to integrate your Flutter app into an existing native iOS or macOS app using Swift Package Manager.

Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any):

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.

[vashworth]

This is tied to the next Flutter release

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 748d2ab):

https://flutter-docs-prod--pr13318-swiftpm-add2app-scklvzuj.web.app

[sfshaza2]

lgtm. Minor nits, but I've also fixed a build failure (due to a moved file) and some long lines of text. If you link to these pages from your blog post, I'm ok with landing this early, once the suggestions are applied.