USWDS-Compile: Use gulp-sass v6 to use modern Sass JS API (Updated)

[mdmower-csnw]

This is a continuation of #140 since additional movement is not expected there. Several changes in that PR were only formatting, which have since been applied in develop.

Summary

Update gulp-sass to v6.0.1 so that the new Sass v3 JS API can be used. See v6 release notes. This is a non breaking change for uswds-compile consumers because the options API is not exposed outside of boolean option sassDeprecationWarnings which is used to configure quietDeps.

See also migration guide from v5 to v6: https://www.npmjs.com/package/gulp-sass#migrating-to-version-6

Related issue

This is a step towards #133 but does not close that issue. A subsequent pull request can build off of this work to expose the modern options API to end users.

Problem statement

For #133, it would be best to avoid exposing the Sass options API until it is modernized, so that users do not initially configure with a deprecated API and then soon after be required to update options again for the modern API.

Solution

This pull request updates gulp-sass to the latest v6 revision and updates internal usage of the options API.

Testing and review

No observable differences should occur. This pull request is successful if no impact is noticed by consumers of uswds-compile.

Dependency updates

Dependency name	Previous version	New version
gulp-sass	5.1.0	6.0.1

[mdmower-csnw]

@annepetersen - Hi Anne, following up on your Slack post, you — yes, you! — can get involved, would now be a good time to nudge maintainers on this pull request, which would open the door for #145 as well?

[annepetersen]

Appreciate the nudge! This one in particular, probably not, but many others could use a hand.

For context, decisions and prioritization have headed in the direction of deprecating USWDS compile rather than expending effort here.