Add pattern(s) for label/form layout

[tylersticka]

This issue was requested by @calebeby as part of this discussion: #1256 (comment)

This label/input relationship in the comment form (#1216) seems to repeat:

This also appears to be the case for the WIP email signup form (#1088):

This suggests that we may want to create a new layout object pattern representing the label and a form field together. In the future, this could be extended to support additional messaging below inputs, but for now the existing use cases are all that's required.

Similar patterns in the wild:

• Primer CSS: Form Group
• Spectrum CSS: Form Item
• Polaris: Form Layout Item
• Grommet: Form Field
• Bolt: Form Field
• Ant Design: Form Item
• Atlaskit: Form Field
• Australian Government: Form Group
• Bootstrap: Form Group
• Clarity: Form Control
• FutureLearn: Form Field Wrapper