feat(css): add support for the subtle color map (#30306)

Issue number: internal

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## What is the current behavior?
The colors are only available in bold colors.

## What is the new behavior?
Adds support for a `subtle` hue to the colors map, generating all of the
same variants but in more subtle colors. This only applies for the
`ionic` theme. Adds an additional variant, `foreground` to be used by
components when the color is being used for text.

The `ios` and `md` themes do not add the `foreground` variant because
this would be a breaking change, requiring all color overrides,
palettes, and new colors to include this variant. I have added tests to
make sure they still work without it.

## Does this introduce a breaking change?
- [ ] Yes
- [x] No

No changes are made to the existing colors.

Author: brandyscarney