fix(tab-button): match height to tab-bar in ionic-md theme (#31209)

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## What is the current behavior?
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Tab buttons in the ionic-md theme are slightly shorter than the tab bar.
This is inconsistent when dev tools are in use in Firefox, causing
screenshot tests to fail.

## What is the new behavior?
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this PR. -->

- Tab buttons completely fill available vertical space by adding a
margin above the button's icon.

## 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. -->
The tab bar content height is 48px while the buttons are 44px high with
the styles specified. Usually the buttons render 44px high, but
sometimes when the button is focused with Firefox dev tools, they render
48px high. When the buttons are made to be 48px high, matching the
container, we don't see this issue.

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Co-authored-by: ionitron <hi@ionicframework.com>
Co-authored-by: Maria Hutt <thetaPC@users.noreply.github.com>

Author: 3 people