USWDS-Compile - Gulp: Support disabling sass sourcemaps

[mdmower-csnw]

• Drop gulp-sourcemaps which is seemingly neglected and is the reason for 3 npm audit warnings. Gulp v4 and v5 have native support for toggling sourcemaps.
• Add new boolean setting settings.compile.sassSourcemaps that defaults to true but can be set to false in consumer projects to disable outputting sass sourcemaps.

This addresses the sass portion of #70. Exclusion of USWDS JS sourcemaps would need a separate setting and handling.

Related issue

Closes #70

[mdmower-csnw]

@mahoneycm @mejiaj - Would you like me to resolve the merge conflicts since #96 was merged, or would you like to handle that?

[mejiaj]

@mdmower-csnw if you're able to resolve with the latest that would help a lot!

[mdmower-csnw]

@mejiaj - When I attempted to test these changes with the current develop branch, I ran into a new issue: #110 . It would be best if that issue is cleaned up in develop before I resolve conflicts in this PR. Let me know if you need any more information there.

[mejiaj]

Thanks for reporting, we'll take a look.

[mdmower-csnw]

@mejiaj - I retested this PR after merging the latest from develop along with #111 to make sure there aren't any regressions with Gulp v5. Things look good from my end.

For a project where I use uswds 3.7.1...

• the generated sourcemap styles.css.map is identical when using Gulp v5 built-in sourcemaps (this PR) compared to using package gulp-sourcemaps (without this PR).
• disabling sass-sourcemaps still works with setting settings.compile.sassSourcemaps.

I've gone ahead and merged upstream develop into this branch because there are no merge conflicts introduced with #110 (this PR and that PR can be merged in any order).

FYI, the merge strategy used in 77305e5 was as follows:

# origin = https://github.com/mdmower-csnw/uswds-compile.git
# upstream = https://github.com/uswds/uswds-compile.git
git checkout csnw-sourcemaps
git reset --hard origin/csnw-sourcemaps
git fetch upstream
git merge upstream/develop
# git reports merge conflict in package-lock.json
git checkout upstream/develop package-lock.json
npm install
git add package-lock.json
git merge --continue