feat: Add config opt 'ignore_install'

This option allows the caller to define a list of conform formatters
that should be ignored during the install process. This might be useful
if the caller already have this tools installed and prefer not to have
them downloaded again by mason.

lua/mason-conform/init.lua

@@ -1,7 +1,15 @@
 local M = {}
 
-function M.setup()
-    require "mason-conform.auto_install" ()
+M.config = {
+    ignore_install = {}
+}
+
+function M.setup(cfg)
+    if cfg and cfg.ignore_install then
+        M.config.ignore_install = cfg.ignore_install
+    end
+
+    require("mason-conform.auto_install")()
 end
 
 return M