addFramework uses incorrect path for Swift libraries

The default path used in the `PBXFileReference` for frameworks with `.tbd` extensions is `/usr/lib`. However, I believe that Swift libraries with the same extension needs to have the path `/usr/lib/swift`.

For example, if you use `addFramework` to add `libswiftAVFoundation.tbd` it produces the following `PBXFileReference` in the project file:
```
27874A9BBF2444AABA64E8E0 /* libswiftAVFoundation.tbd */ = {isa = PBXFileReference; lastKnownFileType = "sourcecode.text-based-dylib-definition"; name = libswiftAVFoundation.tbd; path = usr/lib/libswiftAVFoundation.tbd; sourceTree = SDKROOT; };
```
This will ultimately result in a Crash on iOS versions 12.1 or lower, before the Swift runtime was bundled with the OS.

If you add the library manually in Xcode, the project file has the following `PBXFileReference' added:
```
5DBEA893249434B30034407A /* libswiftAVFoundation.tbd */ = {isa = PBXFileReference; lastKnownFileType = "sourcecode.text-based-dylib-definition"; name = libswiftAVFoundation.tbd; path = usr/lib/swift/libswiftAVFoundation.tbd; sourceTree = SDKROOT; };
```
Note the path difference:
cordova-node-xcode: usr/lib/libswiftAVFoundation.tbd
Xcode: usr/lib/swift/libswiftAVFoundation.tbd

I have an experimental patch [here](https://github.com/jskrepnek/cordova-node-xcode/commit/ebca1c14c11b5897d12e2829f4cd592d00f89d6b) to work around this.

Assuming I'm on the right track here, I'll submit a pull request but I'm hoping that the maintainers can recommend if the approach I used is acceptable or if there's a more effective way to select the correct default path for these libraries than introducing another option.