feat: use custom rollup config

[antoniogiroz]

Allow users to override the default configuration of blueprints, for instance, to add new plugins.

// blueprint.config.js
import postcss from "rollup-plugin-postcss";
import postcssLit from 'rollup-plugin-postcss-lit';

export default {
  library: {
    entryPoints: ['./src/**/index.ts', './src/include/*.ts'],
    externals: [/^tslib/, /^lit/, /^@blueprintui/]
  },
  rollupOptions: {
    plugins: [
      postcss({
        inject: false,
      }),
      postcssLit(),
    ]
  }
};