feat: Customized language picker UI for the onboarding

[g123k]

Hi everyone,

I've tweaked a little bit the UI of the language picker in the onboarding.
It won't stay with the redesigned one, so it's just some minor fixes:

• An edit button, instead of a chevron (it's not a dropdown)
• Really weird paddings
• Same UI as the search bar in the redesign.

The UI is OK both in dark/light modes + the widget used in the preferences is still OK.
I've also checked the a11n is OK.

Some screenshots:
Current UI:

New UI:

[codecov-commenter]

Codecov Report

Attention: 33 lines in your changes are missing coverage. Please review.

Comparison is base (f145c3c) 9.87% compared to head (4f9e308) 9.85%.

Files	Patch %	Lines
...oth_app/lib/pages/onboarding/country_selector.dart	0.00%	19 Missing ⚠️
.../smooth_app/lib/pages/onboarding/welcome_page.dart	0.00%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #4826      +/-   ##
==========================================
- Coverage     9.87%   9.85%   -0.02%     
==========================================
  Files          312     312              
  Lines        15820   15839      +19     
==========================================
- Hits          1562    1561       -1     
- Misses       14258   14278      +20     

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[teolemon]

Looking better 👍