feat: integrate SASS into the repo

[agung2001]

Hi @colineckert 👋
as a promise #137, I create a PR for integrating SASS

Changes

I add 2 files to this PR :

• .gitignore will need this to ignore node_modules and other unnecessary files
• package.json npm package configuration

Feature

now by using SASS all the styles will be stored in style.scss then we can use SASS like syntax for example :

/* H2 styling */
body.h2-underline {
    h2, .HyperMD-header.HyperMD-header-2.cm-line {
        border-bottom: 2px solid var(--background-modifier-border);
        width: 100%;
        padding-bottom: 2px;    
    }
}

Summary

I didn't change README.md cuz I think that's your territory it'll be unpleasant to change someone else README file.
but just for suggestion maybe you want to add some additional sections, so others can contribute to the theme :

## Installation
You can install and compile the styles by running : 
- NPM install `npm install`
- Run Build Script `npm run build`

## Contributing
- You can contribute to this repo by giving a 🌟 to this repo
- or if you want to contribute to the code you might want to check `/styles` directory instead of doing any changes directly to `theme.css` because it'll automatically be replaced when SASS is compiled.

I think that's all from me
wants to know your thought about this
if you have any questions just let me know
Cheers mate 🍻