fix: export TerminalTargetPlatform

The TerminalTargetPlatform enum is used for a  parameter in the terminal
constructor. Correspondingly it should be exported so that it can be
sepcified as parameter.

lib/core.dart

@@ -14,3 +14,4 @@ export 'src/core/input/keys.dart';
 export 'src/core/mouse.dart';
 export 'src/core/state.dart';
 export 'src/terminal.dart';
+export 'src/utils/platform.dart';