feat(select): add --placeholder-opacity, expose shadow parts

[liamdebeasi]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue Number: #17446

What is the new behavior?

• Added --placeholder-opacity and --placeholder-color` css variables
• Added shadow parts for relevant elements

Does this introduce a breaking change?

• Yes
• No

Other information

[brandyscarney]

Testing the UI now

[brandyscarney]

Do we want parts to be prefixed with select? In Segment we just add part="indicator" since the way you will target would be like:

ion-segment::part(indicator)

[liamdebeasi]

👍 I removed the prefix

[brandyscarney]

Approved once changes are made

[gonewayword]

Is this still not working at all for anyone else? I cannot get the color or opacity to change no matter what. I've applied these style variables to the element, parent elements, etc, and no matter what it does not change. Even when the DOM shows it as being there, it's still getting overriden by:

.select-placeholder {
color: currentcolor;
opacity: 0.33;
}

Tried targeting that specific inner div too in a variety of ways, since the shadow dom shows it as "(open)", but nothing either.

Thanks!