chore(textarea): moving ios+md specific margin to native file (#30984)

Issue number: resolves internal

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issue. -->

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->
The Ionic Textarea component, has a margin to avoid the textarea element
to overlap the label when the label placement is stacked. However, as
the ionic theme already has other styles that safeguard the label, this
margin, is causing a misalignment of the content:
<img width="457" height="174" alt="image"
src="https://github.com/user-attachments/assets/f04dce93-57f6-492f-aa54-5f037aaf943f"
/>

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->
- In the new behavior, the corresponding CSS was moved in to the
`textarea.native.scss` file that is inherited by both ios and md themes,
leaving ionic theme unaffected.
- After the change:
<img width="457" height="174" alt="image"
src="https://github.com/user-attachments/assets/ef9cbb22-b5d2-4845-a01d-e0540fe1df70"
/>

## Does this introduce a breaking change?
- [ ] Yes
- [x] No

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## Other information

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screenshots of how the component looks before and after the change. -->

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Co-authored-by: ionitron <hi@ionicframework.com>

Author: rugoncalves