feat(dashboard): Variant edit form (#6870)

**What**
- Adds form to edit variant details.
- If stock and inventory modules are installed we omit inputs for stock and inventory as they should be managed in a separate form in that case. (Still a bit unsure about this, but its what we have in the current admin)

packages/admin-next/dashboard/src/lib/validation.ts

@@ -0,0 +1,34 @@
+import i18next from "i18next"
+import { z } from "zod"
+import { castNumber } from "./cast-number"
+
+/**
+ * Validates that an optional value is an integer.
+ */
+export const optionalInt = z
+  .union([z.string(), z.number()])
+  .optional()
+  .refine(
+    (value) => {
+      if (value === "" || value === undefined) {
+        return true
+      }
+
+      return Number.isInteger(castNumber(value))
+    },
+    {
+      message: i18next.t("validation.mustBeInt"),
+    }
+  )
+  .refine(
+    (value) => {
+      if (value === "" || value === undefined) {
+        return true
+      }
+
+      return castNumber(value) >= 0
+    },
+    {
+      message: i18next.t("validation.mustBePositive"),
+    }
+  )