Fix Websocket Types

[bstka]

Fix Websocket Types: TS1259 because "ws" package using "export =" #283

Test:

$ npm run test:unit && npm run test:typescript

> @fastify/websocket@8.3.0 test:unit
> tap

 PASS ​ test/base.test.js 36 OK 1s
 PASS ​ test/hooks.test.js 39 OK 195.977ms
 PASS ​ test/router.test.js 56 OK 250.149ms


                         
  🌈 SUMMARY RESULTS 🌈  
                         

Suites:   ​3 passed​, ​3 of 3 completed​
Asserts:  ​​​131 passed​, ​of 131​
Time:​   ​3s​
----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------|---------|----------|---------|---------|-------------------
All files |     100 |      100 |     100 |     100 |                   
 index.js |     100 |      100 |     100 |     100 |                   
----------|---------|----------|---------|---------|-------------------

> @fastify/websocket@8.3.0 test:typescript
> tsd

Done in 8.51s.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm