Add profile marker check

[catatsuy]

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Allow users to prevent nvm from automatically updating their shell configuration files by adding '# nvm completions' comment. This follows the same pattern as bun where the presence of the marker indicates the user wants to manage the configuration manually.

When '# nvm completions' is found in .zshrc/.bashrc, the installer will skip adding nvm initialization code, giving users more control over their shell configuration.

https://github.com/oven-sh/bun/issues/7641

refs: #2153 #2076