Dev deps fix + cleanup + refactor

[JackCuthbert]

Sorry for the delay in getting around to some of these fixes, it's been hectic at work lately so I hope this PR makes up for it :slightly_smiling_face:

• I've added a declaration file (and a small change to tslint to allow this) to make type definitions a little easier to use during development
• I've included the development dependencies fix which is based on the work in https://github.com/prisma/serverless-plugin-typescript/pull/157 by @MennoTammens and @jengjeng (thanks!). This should resolve #90 and #154.
• I've refactored the hooks property to better illustrate how the build works now that we're passing in a flag to determine when to package/exclude dependencies
• I've added typescript to the installation instructions in the readme (https://github.com/prisma/serverless-plugin-typescript/issues/151)
• There's also a general cleanup of unused locals, formatting, and style

For those that want to review this (hey @divyenduz :stuck_out_tongue:), the commits should be relatively atomic making it easier if you go through one commit at a time.

[JackCuthbert]

btw is the semantic release still working?

Hmm, doesn't appear to be since the last merge, I think I omitted the semantic-release format on accident :fearful: This one should trigger it

Edit: @divyenduz yes, it's not working. There's an invalid NPM token apparently: https://github.com/prisma/serverless-plugin-typescript/issues/158

[divyenduz]

I will patch it up 👍

Edit: we have a new token now ✅ Please feel free to merge this one :)

[divyenduz]

:tada: This PR is included in version 1.1.8 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: