One JSON plugin list feeds both Nix and vim.pack

To support Nix and non-Nix users of a Neovim config with one plugin list, use JSON: Nix reads it with builtins.fromJSON and maps each entry's nixpkgs attr name to vimPlugins.${name}; init.lua reads it with vim.json.decode and calls vim.pack.add only when pcall(require, "hotpot") (or any bootstrap plugin) fails, meaning no Nix packpath. Encode branches as version and semver ranges as range (wrap in vim.version.range). Neovim has no TOML reader, so TOML would need a hand parser. Tested: plain nvim installed all 41 plugins plus treesitter parsers; the Nix build stayed byte-identical. The user ended up preferring Nix-only for simplicity.