Support passing code as string in addition to file paths

[danbugs]

Currently the SDK only accepts file paths for workload execution. It would be useful to support passing code directly as a string.

Current behavior:

result = await sandbox.run('guest-examples/hello.js')  # File path only

Proposed enhancement:
Add support for passing code as a string, with options:

Option 1: Separate method

result = await sandbox.run_code('console.log("hello")', language='javascript')

Option 2: Auto-detect (file path vs code string)

result = await sandbox.run('console.log("hello")')  # Detects as code
result = await sandbox.run('hello.js')  # Detects as file path

Implementation notes:

• Nanvix runtime currently requires files
• Could create temporary files internally from code strings
• Need to handle cleanup of temporary files
• Should work across all language bindings (NAPI, Python)

Related to PR #20 review from @jsturtevant