feat: standalone visual context CSS generation

[amanabiy]

Issue #, if available:

This change allows new visual contexts to be compiled to separate CSS files that are only included in the bundle when the component is imported, making context addition scalable without growing the base bundle.

Description of changes:

Adds a destination field to the Context interface. When set, the context is excluded from the base styles and compiled to its own CSS file at the specified path. The component that uses the context imports this file directly.

// In the context definition:
{
  id: 'navigation-bar',
  selector: '.awsui-context-navigation-bar',
  destination: 'visual-contexts/navigation-bar.scoped.css',
  defaultMode: 'dark',
  tokens: { ... }
}

// In the component:
import '../visual-contexts/navigation-bar.scoped.css';

Tested with

• components PR #4477 (last commit) consuming directly from this branch
• Generated file verified: correct token overrides, :not(#\9) specificity, excluded from base styles, existing contexts unchanged

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[codecov]

Codecov Report

❌ Patch coverage is 90.56604% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.74%. Comparing base (533392f) to head (3f05583).

Files with missing lines	Patch %	Lines
src/build/tasks/standalone-contexts.ts	50.00%	4 Missing ⚠️
src/shared/declaration/index.ts	97.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #180      +/-   ##
==========================================
- Coverage   96.92%   96.74%   -0.18%     
==========================================
  Files          54       55       +1     
  Lines        1365     1413      +48     
  Branches      246      250       +4     
==========================================
+ Hits         1323     1367      +44     
- Misses         36       40       +4     
  Partials        6        6              

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