Add MacroDef for "Macros 2.0"

Author: jonas-schievink

src/tools/rust-analyzer/lib/ungrammar/rust.ungram

@@ -90,6 +90,7 @@ Item =
 | Impl
 | MacroCall
 | MacroRules
+| MacroDef
 | Module
 | Static
 | Struct
@@ -103,6 +104,11 @@ MacroRules =
   'macro_rules' '!' Name
   TokenTree
 
+MacroDef =
+  Attr* Visibility?
+  'macro' Name args:TokenTree?
+  body:TokenTree
+
 Module =
   Attr* Visibility?
   'mod' Name