feat: add processor css modules support

[marcalexiei]

Close #173

Add support of CSS modules via processor function.

When user return a cssModules property from processor output it will be used as default export from the css file.

Example

sass({
  // ...pluginOptions,
  processor: async (styles, id) => {
    let cssModules = {};
    const postcssProcessResult = await postcss([
      postcssModules({
        getJSON: (_, json) => {
          if (json) cssModules = json;
        },
      }),
    ]).process(styles, {
      from: id,
    });

    return { css: postcssProcessResult.css, cssModules };
  },
});

TODOs

• insert unit test
• test along other named exports
• add documentation