refactor: Specify css-loader options

[SimonAlling]

I think this will make it easier to upgrade css-loader, which in turn I think will be necessary in order to upgrade to webpack 5.

At the time of writing, the latest version of css-loader that matches ^3.2.0 – and the one used in the bootstrapped userscript, SimonAlling/example-userscript@d1d30ea5 and SimonAlling/better-sweclockers@7381699f, according to npm why – is 3.6.0. I went here to check what its default options are, and then I specified them. For some of them, the documented default value isn't obviously correct/sensible:

• hashPrefix

  The documented default value is undefined, but based on the source code, it's actually "". That makes much more sense to me too.

• getLocalIdent

  The documented default value is undefined, but specifying that causes consuming userscripts (e.g. SimonAlling/example-userscript@d1d30ea5) to fail to build with this error:

  TypeError: modulesOptions.getLocalIdent is not a function
  

• localIdentRegExp

  The documented default value is undefined, but based on the source code, it's actually null. Specifying null, however, causes consuming userscripts (e.g. SimonAlling/example-userscript@d1d30ea5) to fail to build with this error:

  Details:
   * options.modules.localIdentRegExp should be one of these:
     string | RegExp
     Details:
      * options.modules.localIdentRegExp should be a string.
      * options.modules.localIdentRegExp should be an instance of RegExp.