Modernize the package - Githubissues

Hey! First of all, sorry if this PR is a little intrusive.

This PR aims to modernize the package as a whole, what it does:

Adds linting (provided by ESLint, using airbnb-base and custom rules)
Converts to ES6 modules, this supports enables using import/from
Adds pnpm support (a very fast and space efficient package manager, used by big packages like Vue)
Updates the README to reflect the changes and apply some best practices.

The underlying code logic has not changed and all tests are passing.

PokeAPI / pokedex-promise-v2