feat(ast): add Swift5 parser module

[phodal]

Summary

• Add new Gradle module :chapi-ast-swift based on Swift 5 ANTLR grammar.
• Provide minimal SwiftAnalyser + listener to extract imports, types (class/struct/enum) and functions.

References

• Swift5 grammar: https://github.com/antlr/grammars-v4/tree/master/swift/swift5

Acceptance criteria

• ./gradlew :chapi-ast-swift:test passes.
• README reflects the new module and language coverage.