chore(button): add changeset

.changeset/nine-kings-repair.md

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+---
+"@spectrum-css/button": major
+---
+
+#### Spectrum 2 migration
+Button now uses Spectrum 2 tokens and specifications, which includes many color changes to all variants. A few other notable changes:
+- Outline buttons are no longer available in accent and negative options — use the filled variant instead.
+- Medium size is now the default. The class `.spectrum-Button--sizeM` is now unnecessary for this size, and has been removed.
+- The `.spectrum-Button--fill` class is no longer needed and has been removed.
+
+The following `--mod` custom properties have been renamed:
+- `--mod-line-height-100` has been renamed to `--mod-button-line-height`
+- `--mod-sans-font-family-stack` has been renamed to `--mod-button-font-family`
+- `--mod-animation-duration-100` has been renamed to `--mod-button-animation-duration`
+- `--mod-bold-font-weight` has been renamed to `--mod-button-font-weight`