Add support for pure CSS Modules

[Timer]

• Operating System: N/A
• Node Version: N/A
• NPM Version: N/A
• webpack Version: N/A
• css-loader Version: 3.2.0

Feature Proposal

Ability to use modules: 'pure', a very-desirable CSS Modules mode.

Feature Use Case

Defining global CSS using CSS Modules is arguably incorrect behavior, which modules: true / modules: 'local' allows.

The underlying postcss-modules-local-by-default library has a more desirable mode called pure, which only allows a :global selector when combined with :local.
This ensures users are not defining global effects within a CSS Modules file.

[alexander-akait]

Pr welcome