feat(utils): Update CSSOM for nested @import computation

[jeeyyy]

This PR does the below:

• handles cssom resolution from nested @import.
  • tackles multiple level nesting @import.
  • tackles cyclic nesting via @import.
  • tackles duplication of @imports across sheets.
  • tackles both cross origin and same origin @import(s).
• updated tests
• using Promise instead of axe.utils.queue

Closes issue:

• CSSOM: Tackle nested @import stylesheets #1054

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• Follows the commit message policy, appropriate for next version
• Has documentation updated, a DU ticket, or requires no documentation change
• Includes new tests, or was unnecessary
• Code is reviewed for security by: Steve

[WilcoFiers]

I think I got most of it. I've asked @stephenmathieson to work with you on refactoring this code. It is too difficult to read, which is going to make it difficult to maintain or update.

[jeeyyy]

Refactoring of CSSOM will be done on the code in develop branch & then this PR will be re-opened.

Updated.

[straker]

Please add unit tests for all the new util functions that have been split out into their own files.

[straker]

@WilcoFiers needs to have final say. Changes look good to me with two caveats:

1. The test to check if a promise is returned will break the tests in any environment where Promise is polyfilled, such as IE 11. If we're OK with that then no change is needed.
2. The unit and integration tests for the main parse stylesheet function tests functionality as a whole set. If we're OK that the separate util functions are not tested individually then no change is needed.