Add :PreProcIfWrap command and C/C++ mappings

Quickly wrap a line or range in `#if 0`/`#endif` to quickly disable
compilation of sections of code.

after/ftplugin/cpp.lua

@@ -1,2 +1,6 @@
 vim.opt.commentstring = '//%s'
 vim.opt.matchpairs:append('<:>')
+
+local opts = { buffer = true, remap = false, silent = true }
+vim.keymap.set('n', '0', ':PreProcIfWrap<CR>', opts)
+vim.keymap.set('v', '0', ':PreProcIfWrap<CR>', opts)