cli: Add support for CSS exports in package build

This adds support for CSS entry points in package exports. When a package
declares a CSS file in its exports field, the CLI will now automatically
bundle it during `backstage-cli package build` and rewrite the export
path from src/ to dist/ at publish time.

The CSS bundling uses lightningcss to resolve @import statements and
preserves @layer declarations that would otherwise be stripped during
bundling.

This allows packages like @backstage/ui to use the standard build command
instead of custom build scripts for CSS bundling.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

yarn.lock

@@ -3377,6 +3377,7 @@ __metadata:
     jest-css-modules: "npm:^2.1.0"
     jsdom: "npm:^27.1.0"
     json-schema: "npm:^0.4.0"
+    lightningcss: "npm:^1.29.1"
     lodash: "npm:^4.17.21"
     mini-css-extract-plugin: "npm:^2.4.2"
     minimatch: "npm:^9.0.0"
@@ -8023,13 +8024,10 @@ __metadata:
     "@tanstack/react-table": "npm:^8.21.3"
     "@types/react": "npm:^18.0.0"
     "@types/react-dom": "npm:^18.0.0"
-    chalk: "npm:^5.4.1"
     clsx: "npm:^2.1.1"
     eslint-plugin-storybook: "npm:^10.3.0-alpha.1"
     glob: "npm:^11.0.1"
     globals: "npm:^15.11.0"
-    lightningcss: "npm:^1.29.1"
-    mini-css-extract-plugin: "npm:^2.9.2"
     react: "npm:^18.0.2"
     react-aria-components: "npm:^1.14.0"
     react-dom: "npm:^18.0.2"
@@ -39285,7 +39283,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"mini-css-extract-plugin@npm:^2.4.2, mini-css-extract-plugin@npm:^2.9.2":
+"mini-css-extract-plugin@npm:^2.4.2":
   version: 2.10.0
   resolution: "mini-css-extract-plugin@npm:2.10.0"
   dependencies: