refactor: Break apart monolithic configuration schema (#1977)

[skip ci]

src/config/schema/elements/stock/custom.ts

@@ -0,0 +1,17 @@
+import { z } from 'zod';
+
+// https://www.home-assistant.io/lovelace/picture-elements/#custom-elements
+export const customSchema = z
+  .object({
+    // Insist that Advanced Camera Card custom elements are handled by other schemas.
+    type: z.string().superRefine((val, ctx) => {
+      if (!val.match(/^custom:(?!advanced-camera-card).+/)) {
+        ctx.addIssue({
+          code: z.ZodIssueCode.custom,
+          message: 'advanced-camera-card custom elements must match specific schemas',
+          fatal: true,
+        });
+      }
+    }),
+  })
+  .passthrough();