add module Appearance (facebook#654)

src/apis/Appearance.bs.js

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+/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */

src/apis/Appearance.md

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+---
+id: apis/AppState
+title: AppState
+wip: true
+---
+
+```reason
+type t;
+
+[@bs.inline "dark"]
+let dark: t;
+
+[@bs.inline "light"]
+let light: t;
+
+[@bs.scope "Appearance"] [@bs.module "react-native"]
+external getColorScheme: unit => option(t) = "getColorScheme";
+
+[@bs.module "react-native"]
+external useColorScheme: unit => option(t) = "useColorScheme";
+```

src/apis/Appearance.re

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+type t = string;
+
+[@bs.inline]
+let dark = "dark";
+
+[@bs.inline]
+let light = "light";
+
+[@bs.scope "Appearance"] [@bs.module "react-native"]
+external getColorScheme: unit => option(t) = "getColorScheme";
+
+[@bs.module "react-native"]
+external useColorScheme: unit => option(t) = "useColorScheme";

src/apis/Appearance.rei

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+type t;
+
+[@bs.inline "dark"]
+let dark: t;
+
+[@bs.inline "light"]
+let light: t;
+
+[@bs.scope "Appearance"] [@bs.module "react-native"]
+external getColorScheme: unit => option(t) = "getColorScheme";
+
+[@bs.module "react-native"]
+external useColorScheme: unit => option(t) = "useColorScheme";