⚡ (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/constants.ts

@@ -0,0 +1,167 @@
+import { Plan } from '@typebot.io/prisma'
+import type { Stripe } from 'stripe'
+
+export const prices = {
+  [Plan.STARTER]: 39,
+  [Plan.PRO]: 89,
+} as const
+
+export const chatsLimits = {
+  [Plan.FREE]: 200,
+  [Plan.STARTER]: 2000,
+  [Plan.PRO]: 10000,
+} as const
+
+export const seatsLimits = {
+  [Plan.FREE]: 1,
+  [Plan.OFFERED]: 1,
+  [Plan.STARTER]: 2,
+  [Plan.PRO]: 5,
+  [Plan.LIFETIME]: 8,
+} as const
+
+export const starterChatTiers = [
+  {
+    up_to: 2000,
+    flat_amount: 0,
+  },
+  {
+    up_to: 2500,
+    flat_amount: 1000,
+  },
+  {
+    up_to: 3000,
+    flat_amount: 1000,
+  },
+  {
+    up_to: 3500,
+    flat_amount: 1000,
+  },
+  {
+    up_to: 4000,
+    flat_amount: 1000,
+  },
+  {
+    up_to: 'inf',
+    unit_amount: 2,
+  },
+] satisfies Stripe.PriceCreateParams.Tier[]
+
+export const proChatTiers = [
+  {
+    up_to: 10000,
+    flat_amount: 0,
+  },
+  {
+    up_to: 15000,
+    flat_amount: 5000,
+  },
+  {
+    up_to: 20000,
+    flat_amount: 4500,
+  },
+  {
+    up_to: 30000,
+    flat_amount: 8500,
+  },
+  {
+    up_to: 40000,
+    flat_amount: 8000,
+  },
+  {
+    up_to: 50000,
+    flat_amount: 7500,
+  },
+  {
+    up_to: 60000,
+    flat_amount: 7225,
+  },
+  {
+    up_to: 70000,
+    flat_amount: 7000,
+  },
+  {
+    up_to: 80000,
+    flat_amount: 6800,
+  },
+  {
+    up_to: 90000,
+    flat_amount: 6600,
+  },
+  {
+    up_to: 100000,
+    flat_amount: 6400,
+  },
+  {
+    up_to: 120000,
+    flat_amount: 12400,
+  },
+  {
+    up_to: 140000,
+    flat_amount: 12000,
+  },
+  {
+    up_to: 160000,
+    flat_amount: 11800,
+  },
+  {
+    up_to: 180000,
+    flat_amount: 11600,
+  },
+  {
+    up_to: 200000,
+    flat_amount: 11400,
+  },
+  {
+    up_to: 300000,
+    flat_amount: 55000,
+  },
+  {
+    up_to: 400000,
+    flat_amount: 53000,
+  },
+  {
+    up_to: 500000,
+    flat_amount: 51000,
+  },
+  {
+    up_to: 600000,
+    flat_amount: 50000,
+  },
+  {
+    up_to: 700000,
+    flat_amount: 49000,
+  },
+  {
+    up_to: 800000,
+    flat_amount: 48000,
+  },
+  {
+    up_to: 900000,
+    flat_amount: 47000,
+  },
+  {
+    up_to: 1000000,
+    flat_amount: 46000,
+  },
+  {
+    up_to: 1200000,
+    flat_amount: 91400,
+  },
+  {
+    up_to: 1400000,
+    flat_amount: 90800,
+  },
+  {
+    up_to: 1600000,
+    flat_amount: 90000,
+  },
+  {
+    up_to: 1800000,
+    flat_amount: 89400,
+  },
+  {
+    up_to: 'inf',
+    unit_amount_decimal: '0.442',
+  },
+] satisfies Stripe.PriceCreateParams.Tier[]