[Feature]: adding CSS options like Postcss and CSS in JS

As this project is on a way towards becoming more flexible and customizable. Its pretty good to give the user an option to use any of the modern + classical styling options Planning to have styling options using the following libraries

• Modern CSS using Postcss
• CSS Modules ( Postcss-css-module or css-loader ) by @wincentrtz

CSS IN JS

• Emotion
• Styled-Component by @wincentrtz
• aphrodite
• linaria
• any other if you want to add Classical Styling Make a way for usual styling using stylesheet and linking it to the html can be done too

Can add all or any or none 😁

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Note, as of v1, these features will be implemented as addons which can be added using mern.json config file, to support the implementation of these addons, after you make changes in your fork, I will be submitting a PR in your fork with the support of making it configurable with mern.json, so please do let me know once you are done with your changes. Try to avoid making changes in the core part of the project and keep it as minimal as possible. like if it's a CSS-in-JS library addition, make changes in the hello component and add the required dependencies you need.