Add a / path join operator for PathJoinSubstitution (backport #868)

[mergify]

Closes robograph-project/planning#18

Adds a PathJoinSubstition(['stuff']) / 'other pattern.

Also adds a PathSubstitution simple wrapper that allows for passing a single value like: PathSubstitution('path') / 'subdir' / PathJoinSubstitution(['some', 'things'])

Make launch feel a little more natural, match the modern pathlib.Path API which we should be encouraging more, I see a lot of os.path in user-facing examples.

Part of ergonomics pass.

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This is an automatic backport of pull request #868 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of e716d1f has failed:

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Changes to be committed:
	modified:   launch/launch/substitutions/__init__.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   launch/launch/substitutions/path_join_substitution.py
	both modified:   launch/test/launch/substitutions/test_path_join_substitution.py

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[ahcorde]

@emersonknapp do you mind to take care of the conflicts?

[ahcorde]

Replaced by #880