Add Prettier configuration file (prettier.config.js) (#3713)

* Create prettier.config.js

* Add comment with .editorconfig

* Remove duplicate settings handled by .editorconfig

* Correct printWidth

* Update prettier.config.js to remove "unknown option" editorconfig: true

prettier.config.js

@@ -0,0 +1,19 @@
+module.exports = {
+  // These settings are duplicated in .editorconfig:
+  tabWidth: 2, // indent_size = 2
+  useTabs: false, // indent_style = space
+  endOfLine: 'lf', // end_of_line = lf
+  semi: false, // default: true
+  singleQuote: true, // default: false
+  printWidth: 100, // default: 80
+  trailingComma: 'es5',
+  bracketSpacing: true,
+  overrides: [
+    {
+      files: '*.js',
+      options: {
+        parser: 'flow',
+      },
+    },
+  ],
+}