Fix TypeScript global declarations: use var instead of let

TypeScript requires 'var' (not 'let') in 'declare global' blocks to properly
augment the global scope across all modules. This fixes build failures where
TypeScript couldn't find ReactOnRails on globalThis.

The previous commit accidentally reverted this change. Now properly using
'var' with the vars-on-top ESLint rule disabled.

Fixes errors:
- Property 'ReactOnRails' does not exist on type 'typeof globalThis'
- Property '__REACT_ON_RAILS_EVENT_HANDLERS_RAN_ONCE__' does not exist

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

packages/react-on-rails/src/context.ts

@@ -1,10 +1,10 @@
 import type { ReactOnRailsInternal, RailsContext } from './types/index.ts';
 
 declare global {
-  /* eslint-disable no-underscore-dangle */
-  let ReactOnRails: ReactOnRailsInternal | undefined;
-  let __REACT_ON_RAILS_EVENT_HANDLERS_RAN_ONCE__: boolean;
-  /* eslint-enable no-underscore-dangle */
+  /* eslint-disable no-underscore-dangle, vars-on-top */
+  var ReactOnRails: ReactOnRailsInternal | undefined;
+  var __REACT_ON_RAILS_EVENT_HANDLERS_RAN_ONCE__: boolean;
+  /* eslint-enable no-underscore-dangle, vars-on-top */
 }
 
 let currentRailsContext: RailsContext | null = null;