Update App.tsx (#133)

Our custom global variable would be nicer declared as a `const`.

`var` doesn't seem to present any advantages here.

Functionality-wise the block scope remains the same as the global scope.

Semantic-wise it's clear it is global as @emin93 already moved it below imports.

Also ib4 wave of ESLint `no-var` adopters (e.g. https://github.com/gabts/eslint-config-react-native-typescript).

Further, see: https://eslint.org/docs/rules/no-var#when-not-to-use-it

template/App.tsx

@@ -26,7 +26,7 @@ import {
   ReloadInstructions,
 } from 'react-native/Libraries/NewAppScreen';
 
-declare var global: {HermesInternal: null | {}};
+declare const global: {HermesInternal: null | {}};
 
 const App = () => {
   return (