feat(schema) add StringEnum to typebox module

docs/api/schema/typebox.md

@@ -269,8 +269,39 @@ const T = {
 }
 ```
 
+##### StringEnum
+
+`StringEnum` is a standalone utility to for specifying an array of allowed string values on a property. It is directly exported from `@feathersjs/typebox`:
+
+```js
+// import the module, first
+import { StringEnum } from '@feathersjs/typebox'
+
+const T = StringEnum(['crow', 'dove', 'eagle'])
+```
+
+To obtain the TypeScript type, use the `Static` utility:
+
+```js
+import { Static } from '@feathersjs/typebox'
+
+type T = Static<typeof T>
+```
+
+```js
+const T = {
+  enum: ['crow', 'dove', 'eagle']
+}
+```
+
 ##### Enum
 
+<BlockQuote>
+
+For string values, use [StringEnum](#stringenum).
+
+</BlockQuote>
+
 ```js
 enum Foo {
   A,

packages/typebox/src/index.ts

@@ -38,6 +38,15 @@ const arrayOfKeys = <T extends TObject>(type: T) => {
   return Type.Unsafe<(keyof T['properties'])[]>({ type: 'array', items: { type: 'string', enum: keys } })
 }
 
+/**
+ * A TypeBox utility that converts an array of provided strings into a string enum.
+ * @param allowedValues array of strings for the enum
+ * @returns TypeBox.Type
+ */
+export function StringEnum<T extends string[]>(allowedValues: [...T]) {
+  return Type.Unsafe<T[number]>({ type: 'string', enum: allowedValues })
+}
+
 export function sortDefinition<T extends TObject>(schema: T) {
   const properties = Object.keys(schema.properties).reduce((res, key) => {
     const result = res as any