Migrate small `/src` top-level files to TS

[quietbits]

Files to migrate

• hashing.js
• jsxdr.js
• network.js
• signing.js
• soroban.js

Checklist

• Migrate files to TS
• In the test/unit directory, add .test.ts files for every file using Vitest where needed (keep the src structure)
• Make sure all tests pass (old JS and new TS)
• Compare the generated vs hand written declarations contain no regressions
• Remove the old JS tests