BBEdit Codeless Language Module for Swift
Keyword, comment, and string highlighting work. Top-level classes, structs, enums, functions, and extensions are indexed and can be folded. Because of limitations in the matching power of codeless language modules, nested declarations are not indexed and are not fold-able.
The basics seem to work well. It doesn’t handle access control keywords yet. A full language module would probably be necessary to handle overloaded methods well.