home: vim: move LSP-related 'on_attach' to 'utils'

As it will be shared between `null-ls` and `lspconfig`, it makes sense
to put it there.

home/vim/lua/ambroisie/utils.lua

@@ -14,4 +14,19 @@ M.is_executable_condition = function(cmd)
     return function() return M.is_executable(cmd) end
 end
 
+-- shared LSP configuration callback
+-- @param client native client configuration
+-- @param bufnr int? buffer number of the attched client
+M.on_attach = function(client, bufnr)
+    -- Format on save
+    if client.resolved_capabilities.document_formatting then
+        vim.cmd([[
+            augroup LspFormatting
+                autocmd! * <buffer>
+                autocmd BufWritePre <buffer> lua vim.lsp.buf.formatting_sync()
+            augroup END
+        ]])
+    end
+end
+
 return M

home/vim/plugin/settings/null-ls.vim

@@ -3,17 +3,7 @@ local null_ls = require("null-ls")
 local utils = require("ambroisie.utils")
 
 null_ls.setup({
-    on_attach = function(client)
-        -- Format on save
-        if client.resolved_capabilities.document_formatting then
-            vim.cmd([[
-            augroup LspFormatting
-                autocmd! * <buffer>
-                autocmd BufWritePre <buffer> lua vim.lsp.buf.formatting_sync()
-            augroup END
-            ]])
-        end
-    end,
+    on_attach = utils.on_attach,
 })
 
 -- C, C++