feat: a11y assistive opportunity (SITES-31943)

[dianaapredaa]

Accessibility - Assistive technology is incompatible on site

Overview
This PR extends the existing accessibility audit worker (from feat-a11y-audit-worker) by adding individual opportunity creation functionality. The enhancement creates URL-specific accessibility opportunities alongside the existing site-wide reports.

Key Features Added

1. Create Individual Opportunities (src/accessibility/utils/generate-individual-opportunities.js) - NEW FILE

• Processes accessibility data to identify issues on individual URLs
• Creates one opportunity that contains multiple suggestions (one suggestion per URL with issues)
• Filters issues to only include tracked accessibility issue types
• Ranks suggestions by total number of issue occurrences per URL
• Manages opportunity lifecycle (cleanup existing, create new)

2. Enhanced Accessibility Handler (src/accessibility/handler.js)
   Added Step 2c: Integration of individual opportunity creation in processAccessibilityOpportunities()

3. New Opportunity Template (src/accessibility/utils/report-oppty.js)
   Added: createAccessibilityAssistiveOpportunity() - Creates 'a11y-assistive' template opportunities
   Purpose: Template for individual accessibility opportunities with assistive technology focus

4. Enhanced Constants (src/accessibility/utils/constants.js)
   Added: new constant for 'a11y-assistive' accessibility opportunity IDs in order to track specific issue types
   Purpose: Defines which accessibility issues should generate individual opportunities

Related Issues: https://jira.corp.adobe.com/browse/SITES-31943

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This PR will trigger a minor release when merged.