Show modified shell in dotnetup to explain modes

[nagilson]

While this is not a complete resolution of #53963 (comment), (it does not fix the UX for 'env' commands), it does change the summary walkthrough to be more clear about the changes you are making and what each dotnetup mode does.

We likely want to enhance the walkthrough further, but I felt this was a large positive net change for only needing a small code change, and the logic here will need to exist in any future walkthrough improvement anyways.

In addition, this makes it more clear for users to understand what files we are about to modify, and whether dotnetup will work at all (example: if they were on an unsupported shell we might still pick terminal mode without any warning.)

The only functional change is the messaging - no behavioral changes were added.

New examples:

Replacement mode (default on Win):
(it also shows if the path is inferred from global.json)

Terminal mode (default on unix):

When you're on an unsupported shell:

Old Behavior:

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[azure-pipelines]

Azure Pipelines:
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There may be pipelines that require an authorized user to comment /azp run to run.

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