feat: add typings to org repo

[jannikkeye]

Motivation: see here

This PR adds a modified version of my typings on DefinitelyTyped to the org repo as that seems to be the consensus now.

[jannikkeye]

I forgot to update the unit test script here, but somehow the tests are failing for me, locally anyway. I tried resetting to a specific commit and one test also fails there. I'm on MacOS and tried different node versions to no success. I tried on commit: 3bb2b07

[mcollina]

npm run coveralls does not exists.

[jannikkeye]

Indeed it does not. Oversight on my part, carryover from another plugin repo.

[mcollina]

One test is a bit flaky, don't worry about it.

[jannikkeye]

I cannot commit my changes because that one test keeps failing.

npm run unit

> [email protected] unit /Users/janisis/code/fastify-multipart
> tap test/*.test.js

test/multipart.test.js .............................. 42/43
  should call finished when both files are pumped
  not ok should be equal
    +++ found                                                           
    --- wanted                                                          
    -2                                                                  
    +0                                                                  
    compare: ===
    at:
      line: 97
      column: 9
      file: test/multipart.test.js
      type: Immediate
      method: _onImmediate
    stack: |
      Immediate._onImmediate (test/multipart.test.js:97:9)
    source: |
      t.equal(fileCount, 2)

total ............................................... 42/43
  

  42 passing (10s)
  1 failing

[mcollina]

git commit -n

[mcollina]

LGTM