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

Author: castastrophe

.changeset/mean-adults-scream.md

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+---
+"@spectrum-css/tokens": minor
+---
+
+Add JSON to token package exports. Remove index files from commit history (continues to build the same assets and ship them in the release). Stop copying the tokens/dist/css/index.css file to tokens/dist/index.css and instead, map dist/index.css in the exports to the same dist/css/index.css file.