Fix Swift 6-mode isolation and attribute warnings

Under SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor these are warnings in Swift 5
mode and errors in Swift 6 mode:

- Drop @retroactive from 'extension LookupType: AppEnum'; LookupType is
  declared in this module, so the attribute never applied.
- Mark SyntaxHighlightTheme and SyntaxHighlighter nonisolated. They were
  inferred @MainActor but are built and used from the nonisolated markdown
  renderers in Task.detached; nonisolated makes the off-main confinement
  correct (Highlightr's JSContext was being driven off the main actor).

0 files changed, 0 insertions, 0 deletions