feat(htmlSanitizer): add allowedTags configuration for safe custom HTML elements

[rupesh-kumar-sah]

Summary of Changes

Resolves #696

This PR adds configurable allowedTags support to htmlSanitizer:

1. Allowlist Merging:
   • sanitizeHtml(dirty, options?: SanitizeOptions) accepts allowedTags: string[], which merges with DEFAULT_ALLOWED_TAGS rather than replacing it.
2. Dangerous Attribute Sanitization:
   • Custom allowed tags (e.g. <mark>, <kbd>, <abbr>) are properly sanitized for dangerous attributes (event handlers like on* and javascript: URLs are stripped).
3. Blocklist Enforcement:
   • Attempting to configure dangerous tags (script, iframe, object, form, etc.) throws a SanitizerConfigurationError.
4. Verification:
   • Comprehensive test suite added in src/__tests__/htmlSanitizer.test.ts (8/8 tests green).
   • Typechecking clean via tsc --noEmit.

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