TSM-04: convert core entry and shared infra to TypeScript

[Justin Hammond (Justintime50)]

Summary

• rename core entry and shared modules to TypeScript: easypost, constants, util, internal_util
• update Vite library entry to src/easypost.ts and resolve extensions for .ts + .js
• keep runtime behavior unchanged while migration proceeds with temporary ts-nocheck on converted files
• allow permissive incremental migration in source typecheck by setting noImplicitAny=false in tsconfig.build.json

Validation

• npm run typescript
• npm run build
• npx eslint src/easypost.ts src/constants.ts src/utils/util.ts src/utils/internal_util.ts
• npx vitest run test/services/module_exports_compat.test.js