Support expressions in confirm attribute

[begoon]

The confirmation prompt accepts any expression, so you can use variables,
concatenation, and function calls:

target := "production"
[confirm("Deploy to " + target + "?")]
deploy:
  echo 'Deploying...'

Recipe parameters can also be used in confirmation prompts:

[confirm("Deploy to " + env + "?")]
deploy env:
  echo 'Deploying to {{env}}...'

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@casey