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.

Start at the addFramework entry point and compare its PBXFileReference path handling with the experimental patch linked in the issue. Reproduce the libswiftAVFoundation.tbd example and confirm that Swift libraries use usr/lib/swift while other frameworks retain their expected paths.

Vom Indexierungsmodell aus dem Issue-Text verfasst.