Create Initial CLI and Logic Structure

[piraz]

Establish the entry point and the internal directory structure for the application logic. This setup follows a flat structure to avoid unnecessary nesting while keeping the CLI interface separated from the core functionality.

Tasks

• Create cmd/nviman/main.go as the application entry point.
• Create manager/ directory to handle Neovim installation and symlinking logic.
• Create api/ directory to handle communication with GitHub releases.
• Implement basic flag parsing (e.g., --version, --help) in main.go.

Acceptance Criteria

• Running go build ./cmd/nviman produces an executable.
• Basic CLI flags return expected placeholder text.
• Directory structure is organized and follows the "no pkg" rule.