Prepend Sass code on build time

[davidballester]

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Short description of the problem:

I cannot prepend Sass code on build time. I either replace everything using the file option or lose the data option, as seen in sass.ts#L137.

What behavior are you expecting?

I expect the data option to behave as with sass-loader, prepending its content to the actual source Sass code.

Steps to reproduce:

1. Create a sass.config.js locally and configure it in package.json, ionic_sass.
2. Export a data option with static content, such as data: "$primaryBase: #ffffff",
3. Use that variable in variables.scss.
4. It will fail on build time, not finding $primaryBase, as it is ignored.

# sass.config.js
const sassConfig = require('@ionic/app-scripts/config/sass.config.js');
const path = require('path');
require('dotenv-defaults').config({
    path: path.join(__dirname, '../.env'),
    defaults: path.join(__dirname, '../.env.defaults'),
    encoding: 'utf-8',
});

module.exports = {
    ...sassConfig,
    data: `$primaryBase: ${process.env.PRIMARY_BASE};`,
};

# variables.scss
$colors: (
    primary: (
        base: $primaryBase,
    )
);

Which @ionic/app-scripts version are you using?
3.2.3

Other information: (e.g. stacktraces, related issues, suggestions how to fix, stackoverflow links, forum links, etc)