Support scoped=True for IncludeLaunchDescription action

[paulsohn]

Description

Currently IncludeLaunchDescription (equivalently, <include> in xml) does not have any scoping option, so it leaks the launch configurations and set environment variables to the caller.

This variable leak is actually a feature that some ROS projects (e.g. Autoware) deliberately exploit to reuse common preset (i.e. the set of variables) -- so the backward compatibility should be kept, while we also need the ability to lint whether this leakage is intentional.

The behavior of <include scoped="true" (...) /> is equivalent to already possible <group scoped="true"><include (...) /></group>.

The implementation is mostly a duplication of group_action.py. The only difference is that I deferred the support of forwarding for now (it acts like forwarding=True.)

Fixes #801 after a discussion I found that #801 also exposes other issues, but nevertheless this PR is highly relevant for what we are missing.
Compared to its own importance, we've been spending too much time on orthogonal issue about how to make forwarding=False / filter what to forward to the includee.

Is this user-facing behavior change?

The change is backward compatible, as IncludeLaunchDescription(scoped=False) is the default behavior. On the other hand the user can specify IncludeLaunchDescription(scoped=True) to make scoped include.

Did you use Generative AI?

The implementation part is done by myself.
Claude Code (Opus 4.6) helped me in generating test cases. A thorough human review involved.

Additional Information

[SuperJappie08]

I'm unsure if this completely addresses #801, as this forwards the external scope.

This unintentional forwarding could also affect the inner launchfile in unintended ways if it is not accounted for.

There are situations where this unintended passing is intended; however, this is not the case described in #801.