Form Item - error and helpertext converted to slots

[Spark450]

Background:

-The use of italics is the GOA standard when referencing the names of legislation

• In order to meet this standard we need to change our support of italics in the form item.

Acceptance Criteria:

• error and helpertext properties also allow slot content
• Update the component documentation to mark properties as slots and make an example of this usage
• Defaults should continue to be "normal" for the form item.