Resolve and install the actual stable release with 'nvimm install stable' and 'nvimm current stable'

[piraz]

Overview

Implement logic in nvimm to resolve and install the actual version set as stable when the user requests nvimm install stable or nvimm current stable.

Requirements

• When nvimm install stable is executed, resolve which release is currently designated as stable (e.g., if 0.11.5 is currently stable, install 0.11.5).
• When a new version becomes stable (e.g., 0.11.6), running nvimm install stable should install 0.11.6.
• When nvimm current stable is used, switch the 'current' symlink to the directory of the current stable version.
• The "stable" marker must reliably follow upstream release status, mapping "stable" keyword to the correct version at installation/switch time.
• The behavior must be consistent for both installing and setting the current version.
• Add clear user feedback and documentation about how the stable resolution works.

Example Flows

• nvimm install stable resolves to (for example) 0.11.5 and installs it.
• If 0.11.6 becomes stable, the same command resolves to 0.11.6 afterward.
• nvimm current stable sets the current symlink to the directory of the current stable version.

Extensibility

• Structure logic for future changes, like beta/rc aliases or custom stability tags.