Add 'jsx' to resolver.sourceExts defaults

Summary:
Widens the default config value for `resolver.sourceExts` to include `.jsx` file extensions. This mirrors us already supporting both `.ts` and `.tsx` out of the box, and [aligns with create-react-app](https://github.com/facebook/create-react-app/blob/f34d88e30c7d8be7181f728d1abc4fd8d5cd07d3/packages/react-scripts/config/webpack.config.js#L356).

Changelog:

**[Breaking]** With a default config, any `.jsx` files will now be resolved before `.json`, `.ts`, `.tsx`

Reviewed By: robhogan

Differential Revision: D37881467

fbshipit-source-id: cb16eb9b190e9f3f5666fd48b11a12feba8a53f1

packages/metro-config/src/__tests__/__snapshots__/loadConfig-test.js.snap

@@ -68,6 +68,7 @@ Object {
     ],
     "sourceExts": Array [
       "js",
+      "jsx",
       "json",
       "ts",
       "tsx",
@@ -224,6 +225,7 @@ Object {
     ],
     "sourceExts": Array [
       "js",
+      "jsx",
       "json",
       "ts",
       "tsx",
@@ -380,6 +382,7 @@ Object {
     ],
     "sourceExts": Array [
       "js",
+      "jsx",
       "json",
       "ts",
       "tsx",
@@ -536,6 +539,7 @@ Object {
     ],
     "sourceExts": Array [
       "js",
+      "jsx",
       "json",
       "ts",
       "tsx",

packages/metro-config/src/defaults/defaults.js

@@ -50,7 +50,7 @@ exports.assetExts = [
 
 exports.assetResolutions = ['1', '1.5', '2', '3', '4'];
 
-exports.sourceExts = ['js', 'json', 'ts', 'tsx'];
+exports.sourceExts = ['js', 'jsx', 'json', 'ts', 'tsx'];
 
 exports.moduleSystem = (require.resolve(
   'metro-runtime/src/polyfills/require.js',