Description

Resolves issue #4142 by adding the medium-negative system-tokens to _spacing-tokens.scss as well as _properties.scss.

Additional information

Input before:

@include u-margin-x("neg-4");

left unexpected output:
messageOriginal: "'neg-4' is not a valid USWDS margin-horizontal token. Valid tokens: 1px, 2px, 05, 1, 105, 2, 205, 3, neg-1px, neg-2px, neg-05, neg-1, neg-105, neg-2, neg-205, neg-3, 4, 5, 6, 7, 8, 9, 10, 15, card, card-lg, mobile, 05em, 1em, 105em, 2em, 0, auto" ╷ 57 │ @return uswds-error( │ ┌───────────^ 58 │ │ "'#{$token}' is not a valid USWDS #{$type} token.#{$valid-token-message}" 59 │ │ ); │ └───^

The error does not occur with this build..

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• [ x ] Follow the 18F Front End Coding Style Guide and Accessibility Guide.
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• [ x ] Title your pull request using this format: [Website] - [UI component]: Brief statement describing what this pull request solves.