v0.4.0

Features

• add shimgen tool for generating alias shims for ROS message packages

Refactoring

• update imports to reflect /internal to /pkg migration in unit tests and implementation
• moved msgs from /internal to /pkg
• update message generation paths to pkg/msgs

Chores

• bump VERSION to 0.4.0 for v0.4.0
• Merge tag 'v0.3.0' into humble