Update to stylelint 16

[bpfoster]

Stylelint 16 had quite a few changes This includes what appears to be the minimally required changes for v16 compatibility. Note that CommonJS is deprecated in stylelint 16. This does not update gulp-stylelint to ESM, but that will need to happen before v17. Deprecation warnings related to CommonJS will be logged to the console. You can disable these via the quietDeprecationWarnings flag, but this seems overly aggressive to me.

The following changes were made in order for v16 compatibility:

• Bump this project version to v15.0.0
• Update node engine to 18.12.0+
  • https://stylelint.io/migration-guide/to-16/#removed-support-for-nodejs-less-than-18120
• Update stylelint-config-standard to a compatible current version
• Replace lintResult.output with lintResult.code
  • https://stylelint.io/migration-guide/to-16/#changed-nodejs-api-returned-resolved-object
• Formatters are now promises; updated to use an async function and await the promise resolution
  • https://stylelint.io/migration-guide/to-16/#changed-nodejs-api-stylelintformatters-object
• color-hex-case rule was previously deprecated and has been removed in v16. Updated unit tests to utilize color-hex-length rule instead.
  • https://stylelint.io/migration-guide/to-15#deprecated-stylistic-rules

fixes #10