feat(picker): add css variables for picker highlight and fade

[brandyscarney]

Issue number: N/A

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What is the current behavior?

Picker has the following CSS variables for styling: --wheel-fade-background-rgb and --wheel-highlight-background

These variables make sense when used from ion-datetime, but picker does not need the wheel prefix.

What is the new behavior?

Datetime

• Adds --wheel-highlight-border-radius
• Keeps --wheel-fade-background-rgb and --wheel-highlight-background
• Assigns these variables to picker's new variables

ion-picker {
  --highlight-background: var(--wheel-highlight-background);
  --highlight-border-radius: var(--wheel-highlight-border-radius);
  --fade-background-rgb: var(--wheel-fade-background-rgb);
}

Picker

Renames --wheel-fade-background-rgb and --wheel-highlight-background and adds another CSS variables to ion-picker:

• Renames --wheel-fade-background-rgb to --fade-background-rgb
• Renames --wheel-highlight-background to --highlight-background
• Adds --highlight-border-radius

Does this introduce a breaking change?

• Yes
• No

Other information

PR to update the Datetime docs

[liamdebeasi]

Had some questions on the naming/how this is used in datetime

[liamdebeasi]

Great job!