Remove basscss dependency

[mejiaj]

Basscss is deprecated and the github repo is archived. We should remove this dependency. Let's use USWDS tokens, utilities, and functions instead.

See main.scss https://github.com/GSA/plainlanguage.gov/blob/030125b923dbe8bf051e4cb3c9f091bd8cd71394/assets/css/main.scss#L10-L13

For example, handling margins https://github.com/basscss/basscss-sass/blob/master/scss/_responsive-margin.scss

Use USWDS tokens for spacing instead https://designsystem.digital.gov/design-tokens/spacing-units/

[nick-mon1]

This will be closed out when site is migrated.