Unblock usage of `stylelint --fix`

[sam-b-rose]

Setup current Sass migrations to be easily portable to stylelint-polaris for using the --fix flag.

• createSassMigrator method to wrap common functionality #7499
• Add stylelint metadata within shim #7541
• More robust stylelint shim to support reporting #7532
• Codify the Suggestion-on-partial-fix pattern #7543

[jesstelford]

There now exists a createSassMigrator factory function which gives us ~90% compatibility with stylelint --fix.