⚡ (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/getChatsLimit.ts

@@ -0,0 +1,19 @@
+import { Plan } from '@typebot.io/prisma'
+import { chatsLimits } from './constants'
+import { Workspace } from '@typebot.io/schemas'
+
+export const getChatsLimit = ({
+  plan,
+  customChatsLimit,
+}: Pick<Workspace, 'plan'> & {
+  customChatsLimit?: Workspace['customChatsLimit']
+}) => {
+  if (
+    plan === Plan.UNLIMITED ||
+    plan === Plan.LIFETIME ||
+    plan === Plan.OFFERED
+  )
+    return -1
+  if (plan === Plan.CUSTOM) return customChatsLimit ?? -1
+  return chatsLimits[plan]
+}