Support Swift files in the .xcodeproj

[tdermendjiev]

*Redirect LD to nsld.sh so we can hook between compiling and linking and generate metadata there (we need appname-Swift.h to be created (and a modulemap for it) so the metadata-generator finds it)
*Set Swift language version
*Set Strip swift symbols to NO
*Add ObjC bridging header so the Swift classes are added to the generated appname-Swift.h

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• All existing tests are passing: https://github.com/NativeScript/ios-runtime/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

What is the current behavior?

Swift files cannot be added to an .xcodeproj and accessed by the NativeScript runtime for several reasons:

• Swift symbols are stripped
• Swift language version is undefined in the project settings
• No metadata is generated for the Swift symbols accessible from the ObjC runtime

What is the new behavior?

After all source files are compiled nsld.sh creates a modulemap pointing to the appname-Swift.h where the generated interface for the Swift classes is. As the modulemap's directory is added in the HEADER_SEARCH_PATHS the metadata-generator (called from nsld.sh) will find the header and generate metadata for it. Finally the linking is executed.

This way all Swift symbols accessible from the ObjC runtime are now accessible from NativeScript.

Implements: #1131