Add support for LESS stylesheets

Created by: hbj

Resolves #10336 (closed)

Changes done has been mainly deduced from the current implementation for SASS support. These include:

• Add opt-in support for LESS and LESS modules to webpack config in react-scripts with the possibility to pass options to less-loader through a predefined file in the app root folder
• Add documentation
• Add LESS support utils to react-dev-utils
• Add tests to kitchensink test suite

Test plan:

• Manually test the end-to-end flow as described in the contribution document. The following features have been tested:
  • Loading of local LESS stylesheets
  • Loading of LESS stylesheets from node_modules
  • Loading of local LESS modules
  • Generated CSS identifier of LESS modules (for index and non-index files)
  • Helper error message output if less hasn't been installed
• Added unit tests and made sure they pass by running the corresponding test suites (yarn e2e:docker --test-suite <test suite>). Tests that are not passing don't seem to be related to this PR.

NOTE: The documentation contains a hint that the feature is available with react-scripts@4.1.0 and higher. I included this temporarily as I don't know in which version this would be included, so this has to be updated once the information becomes available.