Refactor: Add a constants file which exports all constants throughout the application

[anshumanv]

Do you want to request a feature or report a bug?
Discussion for a possible refactor

What is the current behavior?
We're using a bunch of constants throught the code (

webpack-cli/bin/cli.js

Line 21 in 62ab32d

const NON_COMPILATION_ARGS = [

), (

webpack-cli/bin/config-yargs.js

Line 55 in 62ab32d

const CONFIG_GROUP = "Config options:";

) and a lot more. I think it makes sense to bunch them all up in a single file (maybe call constants.js?) and use it wherever needed.

What is the expected behavior?
Single file which exports all constants which is imported whereever needed (a possible refactor).

If this is a feature request, what is motivation or use case for changing the behavior?
Not a feature request.

Please paste the results of webpack-cli info here, and mention other relevant information such as programming language.

webpack-cli info

  System:
    OS: macOS 10.14.3
    CPU: (4) x64 Intel(R) Core(TM) i5-5250U CPU @ 1.60GHz
  Binaries:
    Node: 10.14.1 - /usr/local/bin/node
    Yarn: 1.12.3 - /usr/local/bin/yarn
    npm: 6.4.1 - /usr/local/bin/npm
  Browsers:
    Chrome: 72.0.3626.119
    Safari: 12.0.3
  npmPackages:
    webpack: 4.x.x => 4.29.6
    webpack-dev-server: ^3.1.10 => 3.2.1

Will do a quick PR in case we agree to proceed with this. 😄

[anshumanv]

Will do this after #781 goes through.