Added with-suffix option for pipx inject command

[chrysle]

• I have added an entry to docs/changelog.md

Summary of changes

Added a --with-suffix option to pipx inject command to add the venv suffix to injected apps. Closes #521
Since the pipx uninject command doesn't work with suffixes, maybe an option should also be added there.

Test plan

Tested by running

python3 src/pipx install pycowsay --suffix "_2" 
python3 src/pipx inject pycowsay_2 black --include-apps --with-suffix
nox -s tests-3.8