Create an ESM version of the package

[elchininet]

This pull request migrates de package bundler from Webpack to Rollup and creates an ESM version of it. The new structure of the package is this one:

└── postcss-rtlcss
    ├── esm
    |   └── index.js // esm main module
    |   └── options.js // esm options module
    ├── index.js // cjs main module
    ├── index.d.ts // type definitions main module
    ├── options.js // cjs options module
    └── options.d.ts // type definitions options module

It uses conditional-exports to serve the esm or the cjs version depending on the way of including the package. imports postcssRTLCSS from 'postcss-rtlcss' will load the esm version and const postcssRTLCSS = require('postcss-rtlcss') will load the commonjs one.