USWDS: fix deprecation warnings for global functions - #6746

Summary

Fixes Sass deprecation warnings for length(), unquote(), and unit() by replacing global built-in function calls with their namespaced module equivalents.

Breaking change

This is not a breaking change

Related issue

Closes #6216

Problem statment

Sass is deprecating global built in functions like length, unquote, and unit and moving them to namespaced equivalents. Output of a sass build shows warnings in the terminal and has caused some confusion with downstream consumers.

Solution

Replaced deprecated global function calls with their modern equivalent to fix the root cause of the warnings and prepare for future updates.

Changes

• rem-to-user-em.scss: Replace unit() with math.unit()
• add-bar.scss: Remove unnecessary unquote() calls on interpolated string properties (interpolation already produces unquoted output)
• layout-grid.scss: Replace unquote() with string.unquote(), replace length() with list.length(), add @use "sass:string"
• _grid-container.scss: Replace length() with list.length()

Testing and review

Run npm run test:sass to confirm tests pass. Run npm run build to verify that the only deprecation warnings remaining are releated to the if which will be updated in a subsequent PR.