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Closes #7138
Co-authored-by: Marc Cornellà <marc.cornella@live.com>

Closes #7807
Co-authored-by: Marc Cornellà <marc.cornella@live.com>

This makes it work regardless of where nvm is loaded from. And it uses nvm's
version strings, which distinguish the "system" and "none" NVM environments,
instead of reporting the specific version of the system node.js or erroring,
respectively.

Fixes #4336
Closes #4338

Co-authored-by: Marc Cornellà <marc.cornella@live.com>

Fixes #5453.

* Export $NVM_DIR if it doesn't exist and use it when looking for nvm
* Use $NVM_DIR when looking for nvm for completion
* Use $NVM_DIR when looking for nvm in nvm_prompt_info