feat(neovim): migrate to nixvim with minimal plugins (#260)

Replace bare programs.neovim with programs.nixvim. Only 3 plugins:
neo-tree (file explorer), catppuccin (colorscheme), treesitter.

Add core editor opts matching helix defaults: clipboard, cursorline,
scrolloff, wrap, mouse, undofile, smart search, split direction.
Enable 5 LSP servers: nixd, rust-analyzer, gopls, pyright, bashls.

Signed-off-by: Ryan Yin <xiaoyin_c@qq.com>

flake.lock

@@ -343,9 +343,30 @@
         "type": "github"
       }
     },
+    "flake-parts_4": {
+      "inputs": {
+        "nixpkgs-lib": [
+          "nixvim",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1777932387,
+        "narHash": "sha256-nUYVPiqrzr36ThiQOAr5MKeGHDBSDM3OFWkz0uDjOvc=",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "rev": "71a3a77326609675e9f8b51084cf23d5d1945899",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "type": "github"
+      }
+    },
     "flake-utils": {
       "inputs": {
-        "systems": "systems_3"
+        "systems": "systems_4"
       },
       "locked": {
         "lastModified": 1681202837,
@@ -898,6 +919,28 @@
         "type": "github"
       }
     },
+    "nixvim": {
+      "inputs": {
+        "flake-parts": "flake-parts_4",
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "systems": "systems_3"
+      },
+      "locked": {
+        "lastModified": 1777991353,
+        "narHash": "sha256-DFwjggMV+nzCZpwK6Obxj9F+P59rbLVowGqHETfctBk=",
+        "owner": "nix-community",
+        "repo": "nixvim",
+        "rev": "7986a276960b4dfaed9bb2c3c438b5ba71ae08f1",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "repo": "nixvim",
+        "type": "github"
+      }
+    },
     "nu_scripts": {
       "flake": false,
       "locked": {
@@ -1039,6 +1082,7 @@
         "nixpkgs-master": "nixpkgs-master",
         "nixpkgs-patched": "nixpkgs-patched",
         "nixpkgs-stable": "nixpkgs-stable",
+        "nixvim": "nixvim",
         "nu_scripts": "nu_scripts",
         "nuenv": "nuenv",
         "nur-ryan4yin": "nur-ryan4yin",
@@ -1153,6 +1197,21 @@
         "type": "github"
       }
     },
+    "systems_4": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    },
     "treefmt-nix": {
       "inputs": {
         "nixpkgs": [