Add health checks for nvim-spectre

[nikosavola]

Adds a dedicated health check module for the plugin. Running :checkhealth spectre now verifies:

• Neovim Version: Ensures >= 0.8.
• Dependencies: Validates plenary.nvim presence.
• External Tools: Checks for search (rg, ag) and replace (sed, sd) engines.
• Optional Components: Detects the spectre_oxi Rust module and icon providers (nvim-web-devicons or mini.icons).