feat(extendLocale): new composable

Resolves #3729

src/runtime/composables/defineLocale.ts

@@ -1,5 +1,6 @@
 import { defu } from 'defu'
 import type { Locale, Direction } from '../types/locale'
+import type { DeepPartial } from '../types/utils'
 
 interface DefineLocaleOptions<M> {
   name: string
@@ -12,3 +13,8 @@ interface DefineLocaleOptions<M> {
 export function defineLocale<M>(options: DefineLocaleOptions<M>): Locale<M> {
   return defu<DefineLocaleOptions<M>, [{ dir: Direction }]>(options, { dir: 'ltr' })
 }
+
+/* @__NO_SIDE_EFFECTS__ */
+export function extendLocale<M>(locale: Locale<M>, options: Partial<DefineLocaleOptions<DeepPartial<M>>>): Locale<M> {
+  return defu<Locale<M>, [DefineLocaleOptions<M>]>(options, locale)
+}

src/runtime/types/utils.ts

@@ -1,6 +1,10 @@
 import type { VNode } from 'vue'
 import type { AcceptableValue as _AcceptableValue } from 'reka-ui'
 
+export type DeepPartial<T> = {
+  [P in keyof T]?: T[P] extends object ? DeepPartial<T[P]> : T[P] | undefined
+}
+
 export type DynamicSlotsKeys<Name extends string | undefined, Suffix extends string | undefined = undefined> = (
   Name extends string
     ? Suffix extends string