feat(themes): Add shadcn theme

.changeset/heavy-keys-bow.md

@@ -0,0 +1,5 @@
+---
+'@clerk/themes': minor
+---
+
+Add shadcn theme to @clerk/themes

packages/themes/src/themes/index.ts

@@ -1,4 +1,5 @@
 export * from './dark';
 export * from './shadesOfPurple';
 export * from './neobrutalism';
+export * from './shadcn';
 export * from './simple';

packages/themes/src/themes/shadcn.ts

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+import { experimental_createTheme } from '../createTheme';
+
+export const shadcn = experimental_createTheme({
+  variables: {},
+});