Add a support function to easily get the values of the Placeholders

[jypeter]

Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

I think it would be useful to have an easy way to display the actual values of the Placeholders for a given environment. Something like

placeholders_dic = menuinst.api.get_placeholders_dic('some_env_name')
pprint.pprint(placeholders_dic)

Why is this needed?

• this feature would help debug (or prevent) problems like PY_VER placeholder not defined/updated correctly on Windows #235
• it would make it easier to understand the fine differences between the available placeholders. I have read the Placeholders documentation, but some examples would not hurt. I don't want to create/install lots of dummy json files using the available Placeholders just to check the values
• it would be a support feature of the same kind as Provide "dry-run" functionality #114

What should happen?

No response

Additional Context

It's possible that this feature is already available, but menuinst code is way too fancy for me to find it