Pass computed default value to slate.config.js functions

[t-kelly]

What are you trying to accomplish with this PR?

With the config updates it is possible to set a config value as a function. This function had access to the SlateConfig instance, allowing you to fetch other config values:

// slate-tools.schema.js
module.exports = {
  'webpack.postcss.plugins': (config) => [
    autoprefixer,

    ...(process.env.NODE_ENV === 'production'
      ? [cssnano(config.get('webpack.cssnano.settings'))]
      : []),
  ]
};

However, one thing that was not possible to do was extend the default value of a config item. If the above config was the default value declared, and you wanted to add another PostCSS plugin while keeping the defaults, you would need to redeclare the default values plus add your own:

// slate.config.js
const autoprefixer = require('autoprefixer');
const cssnano = require('cssnano');
const newPostCSSPlugin = require('newPostCSSPlugin');

module.exports = {
  'webpack.postcss.plugins': (config) => [
    newPostCSSPlugin,

    autoprefixer,

    ...(process.env.NODE_ENV === 'production'
      ? [cssnano(config.get('webpack.cssnano.settings'))]
      : []),
  ]
};

This approach requires you to add these libraries as new dependencies in your theme -- not cool.

Now, functions in your slate.config.js file have direct access to the default value, allowing you to extend it without needing to rewrite it:

// slate.config.js
const newPostCSSPlugin = require('newPostCSSPlugin');

module.exports = {
  'webpack.postcss.plugins': (config, defaultValue) => [
    newPostCSSPlugin,
    ..defaultvalue
  ]
};

[lock]

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