perf(bigfile): disable completion to avoid lag when entering insert mode (#2475)

## Description

Default LazyVim uses the extra
[Blink.cmp](https://github.com/saghen/blink.cmp) which creates lag
(multiple seconds in a 600MB `csv` test file) when entering insert mode.

lua/snacks/bigfile.lua

@@ -20,6 +20,7 @@ local defaults = {
       vim.cmd([[NoMatchParen]])
     end
     Snacks.util.wo(0, { foldmethod = "manual", statuscolumn = "", conceallevel = 0 })
+    vim.b.completion = false
     vim.b.minianimate_disable = true
     vim.b.minihipatterns_disable = true
     vim.schedule(function()