Feature Parity with SwiftPM Native Build

[rconnell9]

SourceKit-LSP Integration. Being able to use new build system backend to provide info to SourceKit
REPL Support
Snippets
Symbol graph support
SwiftSDKs

[rconnell9]

SourceKit-LSP repo is live, initial tag created.

• Several PRs open to adopt
• OSS repo is for the BSP protocol that will be adopted by SwiftBuild and SwiftPM.