fix(input, textarea, select): account for multiple start/end slot elements

[liamdebeasi]

Issue number: Internal

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What is the current behavior?

We add margin on content in the start/end slots so they do not run up against either the visible text label of the text in the input. However, we did not account for when multiple elements are placed into the same slot. As a result, this margin is added more times than it needs to be.

What is the new behavior?

• Updated the selector so the margin is only applied to either the last or first element in the slot.

Does this introduce a breaking change?

• Yes
• No

Other information

Test Steps:

1. Check out feature-8.0
2. Add multiple ion-button elements into either the start or end slot on an input (you can reuse the template in src/components/input/test/slots
3. Observe that margin is added to every ion-button.

[sean-perkins]

Changes look good with slotting multiple things to start/end slots 👍