Add VSCode extension recommendations

VSCode on opening a workspace will highlight
recommended extensions based on the a workspace
file at .vscode/extensions.json.

* Create this file and add it as an exception
  to gitignore.
* Add the swift and editorconfig extensions
  as recommendations.

Author: kcieplak

.gitignore

@@ -12,7 +12,9 @@
 xcuserdata
 
 # Ignore VSCode user data.
+!.vscode/
 .vscode/*
+!.vscode/extensions.json
 
 # Commonly used name for build products, at the top level.
 /build

.vscode/extensions.json

@@ -0,0 +1,3 @@
+{
+  "recommendations": ["swiftlang.swift-vscode", "editorconfig.editorconfig"]
+}