Use isAvailable when reading simctl device JSON

`swift-mode:debug-ios-app` was failing because no simulator could be configured; the output of `simctl` was not as expected. The `availability` field of the `device` object had been deprecated and was removed as of Xcode 11 (see https://developer.apple.com/documentation/xcode-release-notes/xcode-11-release-notes heading "Resolved Issues") in favor of a boolean `isAvailable`.

Author: Josh Caswell

swift-mode-repl.el

@@ -431,8 +431,7 @@ or its ancestors."
  (flattened (apply 'seq-concatenate 'list (seq-map 'cdr devices)))
  (available-devices
  (seq-filter
- (lambda (device)
- (string-equal (cdr (assoc 'availability device)) "(available)"))
+ (lambda (device) (cdr (assoc 'isAvailable device)))
  flattened)))
  available-devices))