fix(runtime): don't set gx mapping if already mapped (#24262)

This matches netrw's use of maparg().

runtime/plugin/nvim.lua

@@ -36,9 +36,13 @@ local function do_open(uri)
     vim.notify(err, vim.log.levels.ERROR)
   end
 end
+if vim.fn.maparg('gx', 'n') == '' then
   vim.keymap.set({ 'n' }, 'gx', function()
     do_open(vim.fn.expand('<cfile>'))
   end, { desc = gx_desc })
+end
+if vim.fn.maparg('gx', 'x') == '' then
   vim.keymap.set({ 'x' }, 'gx', function()
     do_open(get_visual_selection())
   end, { desc = gx_desc })
+end