PostCSS preprocessing breaks with Svelte versions 3.30.0+

[arggh]

If I try to define PostCSS plugins when using Svelte version 3.30.0 or later, the build crashes on few different errors, depending on the Svelte version. This error is one of them:

/Users/arggh/.meteor/packages/coffeescript/.1.0.17.jzhn4e.2mhnq++os+web.browser+web.cordova/plugin.compileCoffeescript.os/npm/node_modules/meteor/promise/node_modules/meteor-promise/promise_server.js:116
      throw error;
      ^

TypeError: processed.code.replace is not a function
    at parse_attached_sourcemap (/Users/arggh/Develop/app/node_modules/svelte/src/compiler/utils/mapped_code.ts:305:34)
    at processed_tag_to_code (/Users/arggh/Develop/app/node_modules/svelte/src/compiler/preprocess/index.ts:111:2)
    at process_single_tag (/Users/arggh/Develop/app/node_modules/svelte/src/compiler/preprocess/index.ts:168:10)

Reproduction: https://github.com/arggh/meteor-melte-version-issue/

[arggh]

Currently melte-compiler uses the full return value from postcss as value of code in style preprocessor return value. That used to work, for some reason, before Svelte 3.30.0, but not anymore.

eg.

{
  code: { css: '.example { ... }', postcssStuff, fileInfo etc. }
}

Pull request #4 fixes this by using only the raw css value as value of code.

eg.

{
  code: '.example { ... }'
}