Fix exports for css (#514)

Author: Ben Jervis

.changeset/nice-bobcats-swim.md

@@ -0,0 +1,7 @@
+---
+'@vanilla-extract/css': patch
+---
+
+Export the package.json for css/fileScope
+
+This is required to maintain backwards compatibility with older versions of webpack-plugin

packages/css/package.json

@@ -59,6 +59,7 @@
       "module": "./fileScope/dist/vanilla-extract-css-fileScope.esm.js",
       "default": "./fileScope/dist/vanilla-extract-css-fileScope.cjs.js"
     },
+    "./fileScope/package.json": "./fileScope/package.json",
     "./injectStyles": {
       "browser": {
         "module": "./injectStyles/dist/vanilla-extract-css-injectStyles.browser.esm.js",