feat(tokens): export json, remove duplicate file generation

Author: castastrophe

.changeset/mean-adults-scream.md

@@ -0,0 +1,9 @@
+---
+"@spectrum-css/tokens": minor
+---
+
+Add JSON to token package exports.
+
+Remove index files from commit history (but continue to build and ship them in the releases).
+
+Stop physically copying the `dist/css/index.css` to `dist/index.css` and instead, map `dist/index.css` in the package exports to the same `dist/css/index.css` file.