Ensure browser package does not pollute global namespace (#15978)

Resolves #15977

Our `@tailwindcss/browser` build is intended to run inside a `<script>`
tag inside browsers. Because of how variable assignment within
`<script>` tags work, all variables that were defined within that script
are currently assigned on the global namespace.

This is especially troublesome as eslint uses `$` as a valid mangling
character so importing the CDN build would now redefine `globalThis.$`
which collides with many very popular JavaScript libraries.

In order to avoid polluting the global namespace, this PR changes the
build step to emit an IIFE (so all vars defined are scroped to the
function closure instead of the global namespace).

## Test plan

- Ensure UI tests still work
- <img width="533" alt="Screenshot 2025-01-28 at 16 49 27"
src="https://github.com/user-attachments/assets/1e027451-f58b-4252-bf97-c016a90eb78b"
/>

packages/@tailwindcss-browser/package.json

@@ -3,8 +3,8 @@
   "version": "4.0.0",
   "description": "A utility-first CSS framework for rapidly building custom user interfaces.",
   "license": "MIT",
-  "main": "./dist/index.mjs",
-  "browser": "./dist/index.mjs",
+  "main": "./dist/index.global.js",
+  "browser": "./dist/index.global.js",
   "repository": {
     "type": "git",
     "url": "https://github.com/tailwindlabs/tailwindcss.git",
@@ -19,7 +19,7 @@
     "test:ui": "playwright test"
   },
   "exports": {
-    ".": "./dist/index.mjs",
+    ".": "./dist/index.global.js",
     "./package.json": "./package.json"
   },
   "files": [

packages/@tailwindcss-browser/tsup.config.ts

@@ -1,7 +1,7 @@
 import { defineConfig } from 'tsup'
 
 export default defineConfig({
-  format: ['esm'],
+  format: ['iife'],
   clean: true,
   minify: true,
   entry: ['src/index.ts'],