feat!: compile to JS before publishing

Configure the package to compile TypeScript and copy SCSS files to dist/
before publishing, rather than publishing raw source files. This allows
us to use tsc-alias for @src imports.

Also use a more modern export map to decouple the internal file
structure from the package's API, and add a build step to CI.

BREAKING CHANGE: Consuming projects may need to update their imports or
SASS @use lines.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -35,6 +35,9 @@ jobs:
     - name: Test
       run: npm run test
 
+    - name: Build
+      run: npm run build
+
     - name: Run Code Coverage
       uses: codecov/codecov-action@v5
       with: