Create .gitattributes (#688)

Add a .gitattributes file to easier distinguish source code from
generated code/data. Also, GitHub displays generated files differently.

See
https://github.com/github-linguist/linguist/blob/master/docs/overrides.md#summary

Solves #682

.gitattributes

@@ -0,0 +1,11 @@
+package-lock.json linguist-generated
+packages/docs/site/docs/site/docs/ linguist-documentation
+packages/docs/site/static/diagrams.excalidraw linguist-documentation
+packages/php-wasm/compile/build-assets/zlib/ linguist-vendored
+packages/php-wasm/node/public/ linguist-generated
+packages/php-wasm/node/src/test/test-data/ linguist-generated
+packages/php-wasm/web/public/kitchen-sink/ linguist-generated
+packages/php-wasm/web/public/light/ linguist-generated
+packages/playground/remote/public/wp-* linguist-generated
+packages/playground/remote/src/wordpress/ linguist-generated
+packages/playground/blueprints/public/ linguist-generated