⚡ (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 -->

apps/landing-page/components/PricingPage/FreePlanCard.tsx

@@ -3,7 +3,7 @@ import { HelpCircleIcon } from 'assets/icons/HelpCircleIcon'
 import Link from 'next/link'
 import React from 'react'
 import { PricingCard } from './PricingCard'
-import { chatsLimit } from '@typebot.io/lib/pricing'
+import { chatsLimits } from '@typebot.io/lib/billing/constants'
 
 export const FreePlanCard = () => (
   <PricingCard
@@ -14,9 +14,7 @@ export const FreePlanCard = () => (
         'Unlimited typebots',
         <>
           <Text>
-            <chakra.span fontWeight="bold">
-              {chatsLimit.FREE.totalIncluded}
-            </chakra.span>{' '}
+            <chakra.span fontWeight="bold">{chatsLimits.FREE}</chakra.span>{' '}
             chats/month
           </Text>
           &nbsp;