Support loading XML assets

Summary: Adds support for the `xml` file extension as a loadable asset, and lets Flow treat the type signature as an image

Differential Revision: D58261501

.flowconfig

@@ -62,7 +62,7 @@ munge_underscores=true
 module.name_mapper='^react-native$' -> '<PROJECT_ROOT>/packages/react-native/index.js'
 module.name_mapper='^react-native/\(.*\)$' -> '<PROJECT_ROOT>/packages/react-native/\1'
 module.name_mapper='^@react-native/dev-middleware$' -> '<PROJECT_ROOT>/packages/dev-middleware'
-module.name_mapper='^@?[./a-zA-Z0-9$_-]+\.\(bmp\|gif\|jpg\|jpeg\|png\|psd\|svg\|webp\|m4v\|mov\|mp4\|mpeg\|mpg\|webm\|aac\|aiff\|caf\|m4a\|mp3\|wav\|html\|pdf\)$' -> '<PROJECT_ROOT>/packages/react-native/Libraries/Image/RelativeImageStub'
+module.name_mapper='^@?[./a-zA-Z0-9$_-]+\.\(bmp\|gif\|jpg\|jpeg\|png\|psd\|svg\|webp\|m4v\|mov\|mp4\|mpeg\|mpg\|webm\|aac\|aiff\|caf\|m4a\|mp3\|wav\|html\|pdf\|xml\)$' -> '<PROJECT_ROOT>/packages/react-native/Libraries/Image/RelativeImageStub'
 
 one_sided_type_guards=true