Expose StringJoinSubstitution to frontend

[baconbrot]

This PR exposes StringJoinSubstitution from #843 to the frontend, as suggested by @christophebedard in #843.

Usage

(i.e. with delimiter=. and string components=[https://$(var subdomain), ros, org]):

%YAML 1.2
---
launch:
  - arg:
      name: subdomain
      default: 'wiki'
  - log:
      message: "$(string-join . https://$(var subdomain) ros org)"

results in terminal output

[INFO] [launch.user]: https://wiki.ros.org

[christophebedard]

Other than some minor comments, this looks good.

Would you mind adding tests for frontends, i.e., YAML and XML? See the tests I added to launch_xml and launch_yaml in #802. Also, it would be good to add XML and YAML examples to the Python class docstring.

[christophebedard]

Thanks for adding the tests!

Other than some minor suggestions, this looks great!