CLI: add warning about potential losing of custom stuff added in `.flowershow` before overwriting it

[olayway]

User story

As a power user who makes changes to the template in .flowershow folder, I want to be warned about potentially losing my customizations (e.g. custom document types in contentlayer.config.ts) when running npx flowershow@latest install again.

Acceptance

• [ ] a warning about potentially losing customizations in .flowershow folder after re-running npx flowershow install

Notes

Atm we are asking a user to confirm if he wants to overwrite .flowershow folder when running npx flowershow install again, but we should add some explicit information about losing any changes made to the template.

This is a temporary solution before we figure out a proper upgrade process #324

[rufuspollock]

@olayway for me this would be low priority atm. we hope power users are using git and so can revert 😉 plus upgrading is still a path we need to think about.