generate cgo Objective-C bindings (#56)

* generate cgo Objective-C bindings

Generates cgo code and Go wrappers to call the Objective-C methods. Type information in `api` is generated using `macschema` to fetch and parse from the docs.

`go run ./gen/cmd` generates the `*_objc.gen.go` files containing the cgo code to invoke each Objective-C method, and Go wrappers on top of it.

The generated types are meant to be embedded by the author in a struct to include the generated methods, but allow the author to augment them with additional methods, like:
```
type NSFoo struct { gen_NSFoo }
```

* bump Go test targets to 1.16+

The generator uses `embed.FS` to include the template file, which is new
in Go 1.16.

* filter out "NSView.safeAreaRect" due to build error on GitHub

* also filter "WKWebView.pageZoom" that requires mac 11+

Co-authored-by: Matt Good <mgood@users.noreply.github.com>

.gitattributes

@@ -0,0 +1,2 @@
+api/**/*.json linguist-generated
+generated/**/*.gen.go linguist-generated

.github/workflows/test.yml

@@ -4,7 +4,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go-version: [1.16.x, 1.15.x]
+        go-version: [1.17.x, 1.16.x]
         os: [macos-latest]
     runs-on: ${{ matrix.os }}
     steps:
@@ -15,4 +15,4 @@ jobs:
     - name: Checkout code
       uses: actions/checkout@v2
     - name: Test
-      run: go test -v ./...
+      run: go test -v ./...