fix(types): annotate config API

[oriori1703]

Summary

• explicitly type the forward-declared LuaSnip config API
• prevent setup from being inferred as nullable by EmmyLua
• retain the existing compatibility alias and runtime behavior

This is a narrow annotation workaround for EmmyLuaLs/emmylua-analyzer-rust#1272; the analyzer issue remains open.

Verification

• real require('luasnip').setup {} consumer passes EmmyLua 0.25.1 and LuaLS 3.19.1
• headless Neovim confirms setup and set_config remain the same function
• StyLua and git diff --check pass