⚡ (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/guessIfUserIsEuropean.ts

@@ -0,0 +1,56 @@
+const europeanUnionCountryCodes = [
+  'AT',
+  'BE',
+  'BG',
+  'CY',
+  'CZ',
+  'DE',
+  'DK',
+  'EE',
+  'ES',
+  'FI',
+  'FR',
+  'GR',
+  'HR',
+  'HU',
+  'IE',
+  'IT',
+  'LT',
+  'LU',
+  'LV',
+  'MT',
+  'NL',
+  'PL',
+  'PT',
+  'RO',
+  'SE',
+  'SI',
+  'SK',
+]
+
+const europeanUnionExclusiveLanguageCodes = [
+  'fr',
+  'de',
+  'it',
+  'el',
+  'pl',
+  'fi',
+  'nl',
+  'hr',
+  'cs',
+  'hu',
+  'ro',
+  'sl',
+  'sv',
+  'bg',
+]
+
+export const guessIfUserIsEuropean = () => {
+  if (typeof window === 'undefined') return false
+  return window.navigator.languages.some((language) => {
+    const [languageCode, countryCode] = language.split('-')
+    return countryCode
+      ? europeanUnionCountryCodes.includes(countryCode)
+      : europeanUnionExclusiveLanguageCodes.includes(languageCode)
+  })
+}