Implement switching and displaying the current installed nvim release

[piraz]

Overview

Implement management for the current installed Neovim release in nvimm:

• When the first nvim release is installed, set it automatically as the current.
• New releases installed subsequently are just added locally, not made current.
• To switch between releases, provide the command: nvimm current <release>
• When running nvimm current with no arguments, show the one currently selected as current.
• The "current" release is represented as a symbolic link in the installation directory pointing to the chosen release's directory (e.g., ~/.cache/nvimm/current → ~/.cache/nvimm/<release>).

Requirements

• Automatically set the first installed release as current.
• Only update the "current" symlink when using the current command with a version argument.
• Allow running nvimm current without arguments to display which version is currently set as current.
• Use a symlink (current) in the nvim install path to represent the current version.
• Add safety checks: only link to installed releases, provide errors if a requested version does not exist.
• Document in English, describing behavior of install, switch, and nvimm current output.

Example Flows

• nvimm install 0.11.5 (first): makes 0.11.5 current
• nvimm install 0.12.0 (later): is just installed, not current
• nvimm current: shows which release is marked as current
• nvimm current 0.12.0: switches symlink to 0.12.0 if installed

Extensibility

• Structure logic for possible multi-user or per-project environments in future.