Add nvm enable corepack variable

[smorimoto]

Summary

Add support for the $nvm_enable_corepack variable that automatically enables Corepack when installing Node.js versions.

Changes

• Feature: Add $nvm_enable_corepack variable support
  • When set, automatically runs corepack enable during Node.js installation
  • Follows the same pattern as existing variables like $nvm_default_packages
• Documentation: Update help message and README.md with variable description
• Tests: Add test case to verify the functionality

Usage

# Enable automatic Corepack enablement for all new Node.js installations
set --universal nvm_enable_corepack 1

# Install Node.js (Corepack will be automatically enabled)
nvm install latest

Rationale

This provides a convenient way to ensure Corepack is consistently enabled across Node.js installations, similar to how $nvm_default_packages works for npm packages.

[ljharb]

Why? Corepack isn’t supposed to be enabled by default ever, nor is it even installed in node 25+.