Card: Add .usa-card__body--exdent to scss rules

[btwegmann]

Description

Adds .usa-card__body--exdent to the card exdent scss rules.

Additional information

See #4164

Before you hit Submit, make sure you’ve done whichever of these applies to you:

• Follow the 18F Front End Coding Style Guide and Accessibility Guide.
• Run npm test and make sure the tests for the files you have changed have passed.
• Run your code through HTML_CodeSniffer and make sure it’s error free.
• Title your pull request using this format: [Website] - [UI component]: Brief statement describing what this pull request solves.

[thisisdano]

Thanks for this! I've extended your work in a new PR: #4210
(I've just added the padding rule as well as the margin rule.)
Closing this one.