feat(select): add start and end slots

[averyrousseau]

Issue number: Part of #26297

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What is the current behavior?

With the modern form control syntax, it is not possible to add icon buttons or other decorators to the sides of ion-select, as you can with ion-item.

What is the new behavior?

start and end slots added. While making this change, I also tweaked the CSS selectors responsible for translating the label above the input with "stacked" and "floating" placements, merging this logic into a single class managed by the TSX. I needed to add a new class for whether slot content is present, so the selectors were getting unwieldy otherwise.

Docs PR TBA; I plan on knocking out all three components at once when the features are all complete, to make dev builds easier to manage.

Does this introduce a breaking change?

• Yes
• No

Other information

[liamdebeasi]

Removing Brandy as reviewer in favor of myself cuz she's out 😄

[liamdebeasi]

Two optional things, but otherwise looks good. Great job!