compose menu: Use newly-standard "dynamic import" instead of "require".

This reached Stage 4 in the TC39 process a few months ago:
  https://github.com/tc39/proposal-dynamic-import/

And we already have the Babel plugin for it enabled, via the
`metro-react-native-babel-preset` package.

So, use it to replace the nonstandard `require` borrowed from Node,
making it more visibly parallel to the usual static `import`.

Suggested-by: Anders Kaseorg <anders@zulipchat.com>

src/compose/ComposeMenu.js

@@ -107,8 +107,7 @@ class ComposeMenu extends PureComponent<Props> {
   handleFilePicker = async () => {
     // Defer import to here, to avoid an obnoxious import-time warning
     // from this library when in the test environment.
-    // eslint-disable-next-line global-require
-    const DocumentPicker = require('react-native-document-picker').default;
+    const DocumentPicker = (await import('react-native-document-picker')).default;
 
     let response;
     try {