conform: disable autoformat on save for specified filetypes (#694)

Provide a method to disable autoformat on save lsp fallback for
specified filetypes. By default disable for C/C++ as an example,
because it does not have a well standardized coding style.

Based on conform recipe:
https://github.com/stevearc/conform.nvim/blob/master/doc/recipes.md

init.lua

@@ -632,10 +632,16 @@ require('lazy').setup({
     'stevearc/conform.nvim',
     opts = {
       notify_on_error = false,
-      format_on_save = {
+      format_on_save = function(bufnr)
+        -- Disable "format_on_save lsp_fallback" for languages that don't
+        -- have a well standardized coding style. You can add additional
+        -- languages here or re-enable it for the disabled ones.
+        local disable_filetypes = { c = true, cpp = true }
+        return {
           timeout_ms = 500,
-        lsp_fallback = true,
-      },
+          lsp_fallback = not disable_filetypes[vim.bo[bufnr].filetype],
+        }
+      end,
       formatters_by_ft = {
         lua = { 'stylua' },
         -- Conform can also run multiple formatters sequentially