fix: web SDK build process

[cabljac]

This PR:

1. migrates to tsup for multiple build outputs (ensuring broader bundler/syntax support)

2. adds two simple examples to the SDK

3. switches from a direct dependency on @firebase-scoped packages to a peerDependency on firebase, with the supported versions specified in the package.json
   (ideally should cause less conflicts with package versions)

4. removes an outdated test

[Copilot]

Pull Request Overview

This PR migrates the SDK’s build to tsup with CommonJS and ESM outputs, aligns Firebase dependencies to a peerDependency, and adds both ESM (Vite) and CommonJS (Webpack) example apps.

• Replace all @firebase/* imports with the officially supported firebase/* entry points and stream package outputs through exports in package.json.
• Remove custom postbuild.js version replacer in favor of tsup, and update scripts/devDependencies accordingly.
• Introduce two new example directories (examples/esm and examples/cjs) with full setup, build configs, and README.

Reviewed Changes

Copilot reviewed 24 out of 24 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/product.ts, src/payment.ts, src/init.ts	Switched imports/exports from @firebase/* to firebase/*.
postbuild.js	Removed obsolete script for version replacement.
package.json	Added module/exports, updated scripts, moved Firebase to peer deps and bumped devDeps.
examples/esm/**	Added Vite-based ESM example app (config, source, HTML, README).
examples/cjs/**	Added Webpack CommonJS example app (config, source, HTML, README).
.gitignore	Ignored the generated .tgz and example config files.

Comments suppressed due to low confidence (1)

firestore-stripe-web-sdk/src/init.ts:19

• The version placeholder __VERSION__ is no longer replaced after removing postbuild.js. Integrate version injection into the tsup build (for example via a define plugin) or restore a replacement step so the correct package version is registered at runtime.

registerVersion("firestore-stripe-payments", "__VERSION__");