feat(dropshadow): add dropshadow foundation

[rise-erpelding]

Description

• Adds Storybook documentation (including code snippets and light/dark mode swatches) for Drop Shadow under the Foundations section
  CSS-740

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

• View the Storybook page for Drop Shadow and confirm that it looks accurate according to the design spec (Note that some overrides are applied to the tokens to get dark and light mode working properly on the same page, but there were no issues when applying the tokens to drop shadows i components) - I used this Slack thread with design to inform the initial content/look of the page.

Regression testing

Validate:

1. The documentation pages for at least two other components are still loading, including:

• The pages render correctly, are accessible, and are responsive.

2. If components have been modified, VRTs have been run on this branch:

• VRTs have been run and looked at.
• Any VRT changes have been accepted (by reviewer and/or PR author), or there are no changes.

Screenshots

To-do list

• I have updated relevant storybook stories and templates.
• If my change impacts documentation, I have updated the documentation accordingly.
• ✨ This pull request is ready to merge. ✨

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[jawinn]

Thanks for continuing this exploration of having drop shadows within commons, and setting up the foundations page. Along with using the new tokens.

I'm not sold on the idea of these being a placeholder class within commons, and think that we should just use the four tokens as needed within our CSS, similar to how things played out with down states. For a few reasons:

• It seems a little excessive to import into our components for a single property.
• It may be preferable to use filter: drop-shadow or box-shadow, depending on the situation. I think box-shadow is preferable in most cases, but as you mentioned it may clash with an existing box-shadow in some components. The filter has some downsides, such as showing the shadow around the contours of the alpha channel on a partly transparent image, which might not be wanted.
• With commons there would be the extra generically named mod properties (e.g. --mod-drop-shadow-emphasized-default-x), instead of just having the component specific mod names (if any).

Thoughts on this @pfulton and @castastrophe ?

If we don't use commons, we can still demonstrate the use of these shadow tokens here on the foundations page. Possibly with a code block under each shadow story showing the 4 tokens in a box-shadow property.

[jawinn]

The stories and MDX page look good!

I feel like it would be helpful for users to have a short description for each shadow type and what they're used for, if the design team could provide some language for that. Looking at the internal S2 docs for "Object styles", they had some more info but were called different things (?? -- "Container emphasized", "Elevated", and "Dragged").

[mdt2]

Looking great! +1 to Josh's comment about including some additional details, maybe just a brief blurb about when these would each be used

[pfulton]

Thanks for continuing this exploration of having drop shadows within commons, and setting up the foundations page. Along with using the new tokens.

I'm not sold on the idea of these being a placeholder class within commons, and think that we should just use the four tokens as needed within our CSS, similar to how things played out with down states. For a few reasons:

* It seems a little excessive to import into our components for a single property.

* It may be preferable to use `filter: drop-shadow` or `box-shadow`, depending on the situation. I think `box-shadow` is preferable in most cases, but as you mentioned it may clash with an existing `box-shadow` in some components. The filter has some downsides, such as showing the shadow around the contours of the alpha channel on a partly transparent image, which might not be wanted.

* With commons there would be the extra generically named mod properties (e.g. `--mod-drop-shadow-emphasized-default-x`), instead of just having the component specific mod names (if any).

Thoughts on this @pfulton and @castastrophe ?

If we don't use commons, we can still demonstrate the use of these shadow tokens here on the foundations page. Possibly with a code block under each shadow story showing the 4 tokens in a box-shadow property.

These are good call-outs, and thanks for raising them. I think I'm in agreement with you, @jawinn: not having them in Commons, but definitely having the code examples with some guidance ("use box-shadow when: x, y, z", "use filter for a, b, c") will go a long way for us in communicating how folks should use these.

@rise-erpelding do you think that's do-able in this PR?