feat(bbmodel): 支持转换 Blockbench 方块模型

新增 bbmodel 子命令，将 .bbmodel 转成网易 netease:block_geometry，并支持独立输出目录。

同时导出内嵌 PNG 贴图并在资源包模式下更新 terrain_texture.json，减少手工导入方块模型和材质的重复操作。

Cargo.toml

@@ -12,6 +12,7 @@ clap = { version = "4.5.32", features = ["derive"] }
 uuid = { version = "1.4", features = ["v4", "fast-rng", "macro-diagnostics"] }
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
+base64 = "0.22"
 toml = "0.8"
 zip = "2.2.2"
 walkdir = "2"