feat: Allow users to provide a custom root to resolve files against

[ElMassimo]

Description 📖

This pull request adds root as a user option.

When provided, it takes precedence over Vite's config root.

When not provided, the behavior remains unchanged, so this can be released in a patch version.

Background 📜

In projects where Vite's root is not the project root, it can be preferable for files and dirs provided in the configuration options to be resolved against the project root instead of Vite's root.

Comparison

Before

Files and dirs needed to be resolved manually:

const resolve = (file) => path.resolve(process.cwd(), file)

export default defineConfig({
  plugins: [
    WindiCSS({
      config: resolve('windi.config.ts'),
      scan: {
        dirs: ['app/views', 'app/helpers', 'app/javascript'].map(resolve),
      },
    }),
	...

After

Providing the root allows files and dirs to be resolved to be resolved against it:

export default defineConfig({
  plugins: [
    WindiCSS({
      root: process.cwd(),
      scan: {
        dirs: ['app/views', 'app/helpers', 'app/javascript'],
      },
    }),
	...

Notice that the config option is no longer necessary, as the defaults are now resolved using the provided root.