Implement a few new :Swift* commands

Add commands:

- :SwiftEmitAsm
- :SwiftEmitIr
- :SwiftEmitSil
- :SwiftEmitSil!

With +windows, also add:

- :TabSwiftEmitAsm
- :TabSwiftEmitIr
- :TabSwiftEmitSil
- :TabSwiftEmitSil!