fix: guard auto-setup to not overwrite user config

[AchmadFathoni]

The plugin/render_latex.lua unconditionally calls require("render_latex").setup() when loaded, which resets any user-provided options back to defaults after the user's own setup() call.

This change only runs the auto-setup if the user has not already called setup() themselves.

The guard ensures backward compatibility.

I use vim-plug like

local Plug = vim.fn["plug#"]
vim.call("plug#begin", vim.fn.expand("~/.config/nvim/plugged"))
Plug "techwizrd/render-latex.nvim"
vim.call("plug#end")  -- sources plugin/render_latex.lua → old code calls setup() with defaults

require("render_latex").setup({ install = { auto = false } })

:lua =require("render_latex.config").install.auto return true