Fixes #5845 

Implement tslint for core files

**What is this?**
Implements tslint for both next and next-server, but keeps standardjs/eslint for the .js files that are still there, we're gradually migrating to Typescript.

**How does it work?**
Before every commit (pre-commit) we execute the following `tslint` command:
`tslint -c tslint.json 'packages/**/*.ts`

**TSLint Rules**
In order to avoid as much changes as possible I marked some rules as false. This way we can improve the linter but making sure this step will not break things. (see tslint.json)

**Note**
After merging this PR, you'll need to update your dependencies since it adds tslint to package.json