build: enable DocC integration via CMake

[compnerd]

This pull request updates the build configuration for SourceKit-LSP to add support for a new DocumentationLanguageService and modernizes the CMake setup. The most important changes include introducing a new documentation service library, updating Swift language and CMake requirements, and cleaning up how language services are registered.

Build system modernization:

• Updated CMakeLists.txt to require CMake 3.29, set Swift language version to 6, and add new Swift compiler options for upcoming features. Also, copyright and license headers were added.
• Moved Swift compiler options and the RESILIENT_LIBRARIES definition from Sources/CMakeLists.txt to the top-level CMakeLists.txt for centralization and clarity. [1] [2]

Documentation language service integration:

• Added a new DocumentationLanguageService static library with its own CMake configuration, including all necessary source files and dependencies such as SwiftDocC and DocCCommon.
• Registered DocumentationLanguageService as a dependency of InProcessClient and as a statically known service for Markdown, tutorial, and Swift file types, removing the previous conditional import logic. [1] [2]

Dependency updates:

• Added find_package(SwiftDocC CONFIG REQUIRED) to ensure the documentation compiler is available during build.

[ahoppen]

Thanks for driving this, @compnerd and finally enabling DocC on Windows. Looks good to me assuming this builds but would also like to hear @rintaro’s opinion before merging.

[rintaro]

This looks good to me, thanks!

[compnerd]

@swift-ci please test

[compnerd]

Bleh, this won't build, its missing one change to build.ps1 (to pass the path). I'll upload that in a bit.