⚡ (billing) Automatic usage-based billing (#924)

BREAKING CHANGE: Stripe environment variables simplified. Check out the
new configs to adapt your existing system.

Closes #906





<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
### Summary by CodeRabbit

**New Features:**
- Introduced a usage-based billing system, providing more flexibility
and options for users.
- Integrated with Stripe for a smoother and more secure payment process.
- Enhanced the user interface with improvements to the billing,
workspace, and pricing pages for a more intuitive experience.

**Improvements:**
- Simplified the billing logic, removing additional chats and yearly
billing for a more streamlined user experience.
- Updated email notifications to keep users informed about their usage
and limits.
- Improved pricing and currency formatting for better clarity and
understanding.

**Testing:**
- Updated tests and specifications to ensure the reliability of new
features and improvements.

**Note:** These changes aim to provide a more flexible and user-friendly
billing system, with clearer pricing and improved notifications. Users
should find the new system more intuitive and easier to navigate.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/lib/billing/formatPrice.ts

@@ -0,0 +1,21 @@
+import { guessIfUserIsEuropean } from './guessIfUserIsEuropean'
+
+type FormatPriceParams = {
+  currency?: 'eur' | 'usd'
+  maxFractionDigits?: number
+}
+
+export const formatPrice = (
+  price: number,
+  { currency, maxFractionDigits = 0 }: FormatPriceParams = {
+    maxFractionDigits: 0,
+  }
+) => {
+  const isEuropean = guessIfUserIsEuropean()
+  const formatter = new Intl.NumberFormat(isEuropean ? 'fr-FR' : 'en-US', {
+    style: 'currency',
+    currency: currency?.toUpperCase() ?? (isEuropean ? 'EUR' : 'USD'),
+    maximumFractionDigits: maxFractionDigits,
+  })
+  return formatter.format(price)
+}