Prefer const wherever possible

Summary:
Non-final identifiers make code harder to understand.
This is particularly true for JavaScript where even the *type*
can change as a value gets reassigned later.

This enforces to use `const` whereever possible, but doesn't
"outlaw" `let`. Mixed destructuring is also still allowed.

Used `eslint --fix` to change all existing cases.

Reviewed By: jknoxville

Differential Revision: D16131329

fbshipit-source-id: 2eceaca7c603b71b36e005be5d135e1849f2518d

static/transforms/electron-stubs.js

@@ -5,10 +5,10 @@
  * @format
  */
 
-let babylon = require('@babel/parser');
-let fs = require('fs');
+const babylon = require('@babel/parser');
+const fs = require('fs');
 
-let electronStubs = babylon.parseExpression(
+const electronStubs = babylon.parseExpression(
   fs.readFileSync('static/electron-stubs.notjs').toString(),
 );