refactor(fieldlabel): spectrum-Form cleanup and fixes

[jawinn]

Description

.spectrum-Form cleanup and fixes (this CSS exists within the fieldlabel component)

• Fixes missing spacing between form items in Labels Above variant. This was using the wrong token (top-to-asterisk)
• Removes --spectrum-field-label-top-to-asterisk as it is no longer used
• Cleans up the CSS and its custom properties a bit. Adds some additional -mod properties.
• Removed two comments that were no longer applicable. One mentioned a docs file that does not seem to exist anymore, and the other mentioned a token value of 20px but it didn't equal 20px.
• Fixes form negative margin being too large and it moving outside its container. Instead use same value as border-spacing to remove the extra space, to make this like a margin top and bottom of zero which seems like the intention.
• Adds Form stories to Storybook (previously did not exist within Storybook)
• Fixes some incorrect markup around field group in the examples

How and where has this been tested?

Please tag yourself on the tests you've marked complete to confirm the tests have been run by someone other than the author.

Validation steps

• Standard variant looks the same in example docs
• Margins between items in Labels above variant looks correct (no longer smushed) @jawinn I confirmed this one with the design team
• Negative margin on Standard form equals the border spacing; content no longer overflows its container (screenshot below shows previous behavior)
• Two new stories appear for "Form" and look okay
• Adjusted example markup around field group looks correct

Regression testing

Validate:

1. A legacy documentation page (such as accordion), including:

• The page renders correctly
• The page is accessible
• The page is responsive

2. A migrated documentation page (such as action group), including:

• The page renders correctly
• The page is accessible
• The page is responsive

Screenshots

Negative margin bug in prod (negative margin too large):

To-do list

• I have read the contribution guidelines.

• I have updated relevant storybook stories and templates.

• I have tested these changes in Windows High Contrast mode.

• If my change impacts other components, I have tested to make sure they don't break.

• If my change impacts documentation, I have updated the documentation accordingly.

• ✨ This pull request is ready to merge. ✨

[github-actions]

🚀 Deployed on https://pr-2173--spectrum-css.netlify.app

[mdt2]

LGTM! We talked offline about getting the space between fields validated by design, but they are looking more spacious to me.

[jawinn]

LGTM! We talked offline about getting the space between fields validated by design, but they are looking more spacious to me.

I checked with the design team and they're good with that updated spacing on the "Labels Above" variant.