Add Turbopack loader support and automatically adapt to Webpack

[refirst11]

• I have followed (at least) the PR section of the contributing guide.

Overview

Closes #6
Implemented turbopack-loader.ts and integrated it into pigment-css-nextjs-plugin/src/index.ts with hybrid Webpack/Turbopack support.

Changes

• Removed the branching within withPigment in packages/pigment-css-nextjs-plugin/src/index.ts.
• Modified to simultaneously set up nextConfig.webpack for Webpack and nextConfig.turbopack for Turbopack, always returning an object containing both settings.

Test and Verification

Confirmed that startup, build, and style application work correctly in the following environments.

examples/pigment-css-nextjs-ts:

• pnpm dev (Turbopack default startup) -> Starts successfully, style applied (confirmed)
• pnpm dev --webpack (Webpack forced startup) -> Starts successfully, style applied (confirmed)
• pnpm build -> Build succeeds without errors

How to verify locally

1. Temporarily update pnpm-workspace.yaml to include "examples/*":

2. In examples/pigment-css-nextjs-ts/package.json, change @pigment-css/nextjs-plugin from "latest" to "workspace:^":

3. Run pnpm dev, pnpm build, pnpm start in both Turbopack and Webpack mode.

Styles may not be installed due to the cache in the .next file. If your .next file is old, please delete it before starting the application.