Seedwidget programatic placement

[finetjul]

Context

VTK.js is lacking an example to programmatically place a widget.

Results

Improve the seed widget API and enhance its example.

Changes

• Documentation and TypeScript definitions were updated to match those changes

PR and Code Checklist

• semantic-release commit messages
• Run npm run reformat to have correctly formatted code

Testing

• This change adds or fixes unit tests
• Tested environment:
  • vtk.js: master
  • OS: Windows 11
  • Browser: Chrome

[daker]

@finetjul not sure if it's related but the last release job failed:

2026-01-04T22:44:25.0308531Z Firefox 146.0 (Linux x86_64) INFO: 'Test Suite: Test HardwareSelector'
2026-01-04T22:44:25.0309492Z Firefox 146.0 (Linux x86_64) INFO: '	[PASS] vtkHardwareSelector TestHardwareSelector'
2026-01-04T22:44:25.0310749Z Firefox 146.0 (Linux x86_64) INFO: '	[FAIL] Hardware selector takes less than ten normal renders (1654, 134, 1586)'

[finetjul]

@finetjul not sure if it's related but the last release job failed

This is unrelated, the Hardware Selector test does not use the SeedWidget therefore this commit can't impact the test.