Add all types in the utils submodule as package-level exports (#25)

## Motivation

`snaps` repos have continued to expand its usage of `utils` types in the
`superstruct` package.

Unfortunately, the workaround of importing from `superstruct/dist/utils`
is no longer available in the `@metamask/superstruct` fork, so importing
new `utils` types will require new version releases going forward.

To prevent this from blocking downstream development, this commit adds
all currently available types in the `utils` submodule as package-level
exports.

## Description

- Newly exports the following types: `Assign`, `If`, `IsUnion`,
`ObjectType`, `PartialObjectSchema`, `StructSchema`, `TupleSchema`

## References

- Closes #26
- Blocking MetaMask/snaps#2445
  - Blocking MetaMask/snaps#2514

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Newly exports the following types: `Assign`, `If`, `IsUnion`, `ObjectType`, `PartialObjectSchema`, `StructSchema`, `TupleSchema` ([#25](https://github.com/MetaMask/superstruct/pull/25)).
+
 ## [3.0.0]
 
 ### Added

src/index.ts

@@ -6,16 +6,23 @@ export * from './structs/types.js';
 export * from './structs/utilities.js';
 export type {
   AnyStruct,
+  Assign,
   EnumSchema,
+  If,
   InferStructTuple,
   IsExactMatch,
   IsMatch,
   IsRecord,
   IsTuple,
+  IsUnion,
   ObjectSchema,
+  ObjectType,
   OmitBy,
   Optionalize,
+  PartialObjectSchema,
   PickBy,
   Simplify,
+  StructSchema,
+  TupleSchema,
   UnionToIntersection,
 } from './utils.js';