Move @types/express to dependencies

[Methuselah96]

Description

The outputted types depend on @types/express in types.d.ts and therefore the package should have a dependency on @types/express instead of just a devDependency.

Motivation and Context

Without the dependency it causes this error:

../../.yarn/cache/http-proxy-middleware-npm-2.0.1-d5d4a98393-0de65bc664.zip/node_modules/http-proxy-middleware/dist/types.d.ts:6:31 - error TS2307: Cannot find module 'express' or its corresponding type declarations.

6 import type * as express from 'express';
                                ~~~~~~~~~


Found 1 error.

How has this been tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.