fix: reduce dead code false positives for Next.js and TS imports

[DaisukeYoda]

Summary

• resolve alias imports in dead code aggregation to avoid false positives on aliased paths
• treat Next.js App Router convention exports (for example default and generateMetadata) as framework-consumed
• improve TypeScript handling for dead-code import detection by recognizing type_identifier and skipping explicit import type lines
• add regression tests for alias imports, Next.js reserved exports, and TS type-import/type-reference usage

Validation

• go test ./internal/analyzer
• go test ./...
• go run ./cmd/jscan analyze --select deadcode --json ../codescan/src (before: 8 findings, after: 0 findings)