fix(picker): add separate mod for label font-weight

[mlogsdon18]

Description

CSS-573

This adds a font-weight property with a mod to the .spectrum-Picker-label class. There is a font-weight mod that already existed for the placeholder (if the label also had the .is-placeholder class) so I also updated that mod to be --mod-picker-placeholder-font-weight so that they stayed separate.

In testing this I also realized that the Picker storybook did not implement customStyles so I added that ability too. I also renamed the internal component of PickerButton to Picker to avoid confusion with the actual Picker Button component.

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.

1. Open the docs for the Picker component:
   - [ ] Inspect the page and add a --mod-picker-font-weight: 800 to the first example.
   - [ ] Verify that the placeholder font-weight does not change
   - [ ] Add the same mod --mod-picker-font-weight: 800 to the Open example
   - [ ] Verify that the font weight for the selected value inside the Picker changes

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

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]

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[Westbrook]