Add toggleContent() method to Dropdown

[WarLikeLaux]

Q	A
Is bugfix?	❌
New feature?	✔️
Docs added?	❌
Tests added?	✔️
Breaks BC?	❌

What does this PR do?

Adds toggleContent(string|Stringable $value) method to Dropdown that sets custom content for the toggle button instead of using the item label.

Use cases

When the dropdown toggle should display something other than plain text - an icon, avatar image, or any Stringable widget:

// Icon toggle
Dropdown::widget()
    ->toggleContent((new Span())->content('🔔'))
    ->toggleAttributes(['class' => 'btn btn-light', 'data-bs-toggle' => 'dropdown'])
    ->items([...])

// Plain text override
Dropdown::widget()
    ->toggleContent('Menu')
    ->items([...])

Without toggleContent(), the toggle button renders the item's label as before.

No BC break: new optional method, existing code unaffected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (fe1a223) to head (6182334).

Additional details and impacted files

@@             Coverage Diff             @@
##              master      #153   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity       327       329    +2     
===========================================
  Files              8         8           
  Lines           1038      1044    +6     
===========================================
+ Hits            1038      1044    +6     

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[vjik]

@WarLikeLaux resolve conflicts, please

[vjik]

What about adding support Stringable objects to label and respect NoEncodeInterface?

Seems, it resolves task without adding new methods.

[samdark]

@WarLikeLaux added a test for label leaks. Please ensure the case is handled properly.