fix: output builds for all syntax with tsup

[cabljac]

No description provided.

[Copilot]

Pull Request Overview

This PR replaces the existing TypeScript build setup with a tsup‐based workflow, ensures both CommonJS and ESM bundles are emitted, and updates package metadata and dependency versions.

• Introduces tsup.config.ts for unified CJS/ESM output and in‐bundle version injection
• Updates tsconfig.json to target ES2018 and use ESNext modules
• Removes the old postbuild.js, adjusts package.json main/module/exports, scripts, and bumps dependency/devDependency versions

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
tsup.config.ts	Added tsup configuration for CJS/ESM bundles, source maps, and version
tsconfig.json	Changed target to ES2018, module to ESNext, cleaned up compiler options
postbuild.js	Deleted obsolete post-build version replacement script
package.json	Updated entry points, exports, scripts, and moved/bumped dependencies

[Copilot]

The @firebase/app package has been moved to devDependencies but is required at runtime by consumers. It should be listed under dependencies or peerDependencies to ensure it’s installed for production use.

Suggested change

	"@firebase/app": "^0.13.0",
	"@microsoft/api-documenter": "^7.13.65",
	"@microsoft/api-extractor": "^7.18.17",
	"@microsoft/api-documenter": "^7.13.65",
	"@microsoft/api-extractor": "^7.18.17",
	"@microsoft/api-extractor": "^7.18.17",