Swift: first skeleton extractor

This adds a first dummy extractor for swift.

Running `bazel run //swift:install` will create an `extractor_pack`
directory in `swift`. From that moment providing `--search-path=swift`
will pick up the extractor.

.bazelrc

@@ -0,0 +1,3 @@
+build --copt="-std=c++17"
+
+try-import %workspace%/local.bazelrc