Align build config with pcui

[willeastcott]

Summary

• Replace rollup-plugin-postcss with rollup-plugin-sass (using api: 'modern'), matching pcui's build approach and eliminating the Dart Sass legacy JS API deprecation warning
• Add sourcemap: true to the umd and module rollup output configs, fixing the @rollup/plugin-typescript sourcemap warning
• Remove the unused postcss plugin from the umd and module targets (SCSS is only imported in the dedicated styles entry point)
• Net reduction of 74 transitive dependencies

[Copilot]

Pull request overview

This PR aligns the library’s Rollup build pipeline with the approach used in pcui, primarily by switching the stylesheet build to rollup-plugin-sass (modern Sass API) and cleaning up sourcemap/plugin configuration across targets.

Changes:

• Replace rollup-plugin-postcss with rollup-plugin-sass for the dedicated styles entry point (modern Sass API) to eliminate the Dart Sass legacy JS API deprecation warning.
• Enable sourcemap: true for the UMD and ESM outputs to resolve the @rollup/plugin-typescript sourcemap warning.
• Remove PostCSS processing from the UMD/ESM targets (SCSS is only imported via the separate styles build).

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
rollup.config.mjs	Switch styles pipeline to rollup-plugin-sass, remove PostCSS from JS bundles, and enable sourcemaps for UMD/ESM outputs.
package.json	Swap devDependency from rollup-plugin-postcss to rollup-plugin-sass.
package-lock.json	Lockfile updates reflecting the dependency swap and reduced transitive dependency set.

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