feat: add support for mapped types

[bmv437]

Issue #, if available: AWSUI-61054

Description of changes:

Motivation

The documenter crashes when a component interface uses a mapped type (e.g. { [name in IconProps.Name]?: ReactNode }), because mapped type members are synthetic symbols and getDeclarations() returns no usable property node. This blocks replacing 140+ hand-listed properties in IconProviderProps.Icons with a single mapped type (AWSUI-61054).

Changes

• Add tryExtractDeclaration and isOptionalSymbol helpers to type-utils.ts for handling synthetic symbols from mapped types
• Update extractor.ts and object-definition.ts to gracefully handle properties that lack PropertySignature/PropertyDeclaration nodes, falling back to checker.getTypeOfSymbolAtLocation() and symbol flag checks
• Add test fixtures and tests covering mapped types via union constraint, Record<K,V>, and inline mapped types

Testing

• New mapped-types.test.ts passes which covers property names, types, and optionality.
• All existing tests pass with no regressions
• Smoke-tested against the components repo with IconProviderProps.Icons converted to a mapped type, and documenter output is identical to the manually listed version

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.38%. Comparing base (c76c5a2) to head (ddc761c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #111      +/-   ##
==========================================
+ Coverage   96.75%   97.38%   +0.62%     
==========================================
  Files          11       11              
  Lines         864      918      +54     
  Branches      290      294       +4     
==========================================
+ Hits          836      894      +58     
+ Misses         28       24       -4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[SpyZzey]

Could you add a test that covers these lines (for when declaration is undefined/null)?

[bmv437]

Just pushed changes that add the missing coverage.