feat(lsp): support completion itemDefaults

test/functional/plugin/lsp/completion_spec.lua

@@ -248,4 +248,32 @@ describe('vim.lsp._completion', function()
     item.user_data = nil
     eq(expected, item)
   end)
+
+  it('uses defaults from itemDefaults', function()
+    --- @type lsp.CompletionList
+    local completion_list = {
+      isIncomplete = false,
+      itemDefaults = {
+        editRange = {
+          start = { line = 1, character = 1 },
+          ['end'] = { line = 1, character = 4 },
+        },
+        insertTextFormat = 2,
+        data = 'foobar',
+      },
+      items = {
+        {
+          label = 'hello',
+          data = 'item-property-has-priority',
+          textEditText = 'hello',
+        },
+      },
+    }
+    local result = complete('|', completion_list)
+    eq(1, #result.items)
+    local item = result.items[1].user_data.nvim.lsp.completion_item --- @type lsp.CompletionItem
+    eq(2, item.insertTextFormat)
+    eq('item-property-has-priority', item.data)
+    eq({ line = 1, character = 1 }, item.textEdit.range.start)
+  end)
 end)