feat: plugin nativescript.config support with SPMPackage inclusion

[NathanWalker]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• Tests for the changes are included.

What is the current behavior?

Plugins cannot define their own Swift packages.

What is the new behavior?

Plugins can now include a nativescript.config.ts at root of plugin directory to have any SPMPackages included in the build.