updated getting started

[heckj]

Update of the Getting Started guide for Swift Package Manager

Motivation:

The existing getting started guide focused entirely on dependency resolution, with content that pre-dates swift testing, dependency CLI additions, and common use of templates to start packages.

Modifications:

Revised the getting started guide article, using recent updates to example packages that were previously created, and recently updated to reflect swift-testing and use Swift 6.0 by default.

Result:

• focus on starting with swift package init
• shows command line additions of dependencies alongside changes to Package.swift manifest file
• include source AND tests (using swift-testing)
• walk through swift build, swift test, and swift package resolve
• illustrate using transitive dependencies using the existing dealer example package
• show using swift run for executable targets

[heckj]

@swift-ci please test