feat: export browser integration from "msw/browser"

[kettanaito]

• Addresses the browser-side type definitions concerns from feat: split the library into "core", "browser" and "node" #1584

Changes

• Browser integration (setupWorker and its types) is now exported under msw/browser.
• The msw root-level export only exports primitives and utilities. No integrations there.

Why this change?

I want to ship proper ESM support and CJS support for older bundlers. With ESM it's pretty clear, and it's been addressed in #1584. The problem is TypeScript. With conditional exports, TS will not know what environment is the module you're writing:

// src/pages/index.tsx
import { setupWorker } from 'msw'
// No exported thingie "setupWorker" in "msw"!

Because TS will assume Node and will resolve msw under Node module resolution, respecting its exports, where no setupWorker types are exported in lib/node/index.d.ts (rightfully).

To ship this CJS/ESM support, we must provide the export pattern that will not make TypeScript confused. Instead of diving into another half a year of work to restructure this into a monorepo, I'm falling back to a /browser directory export:

import { rest } from 'msw'
import { setupWorker } from 'msw/browser'

• Explicit intention.
• CJS-compatible.
• Old bundler-compatible (via the root-level /browser directory).
• TypeScript is happy (non-ambiguous import).

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