feat: Add a builder for variants

config/variants.ts

@@ -0,0 +1,33 @@
+import { ConfigSpec } from "../types.ts";
+import { BuilderExtract, IBuilder } from "./builder.ts";
+import { Config } from "./mod.ts";
+
+export class Variants<A extends { [key: string]: ConfigSpec }> extends IBuilder<A> {
+  static of<
+    A extends {
+      [key: string]: Config<ConfigSpec>;
+    }
+  >(a: A) {
+    // deno-lint-ignore no-explicit-any
+    const variants: { [K in keyof A]: BuilderExtract<A[K]> } = {} as any;
+    for (const key in a) {
+      // deno-lint-ignore no-explicit-any
+      variants[key] = a[key].build() as any;
+    }
+    return new Variants(variants);
+  }
+
+  static empty() {
+    return Variants.of({});
+  }
+  static withVariannt<K extends string, B extends ConfigSpec>(key: K, value: Config<B>) {
+    return Variants.empty().withVariant(key, value);
+  }
+
+  withVariant<K extends string, B extends ConfigSpec>(key: K, value: Config<B>) {
+    return new Variants({
+      ...this.a,
+      [key]: value.build(),
+    } as A & { [key in K]: B });
+  }
+}