+ add blocknilarg for prism (#1087)

See https://bugs.ruby-lang.org/issues/19979

```
def foo(&nil)
end
```

Author: Earlopain

doc/AST_FORMAT.md

@@ -1145,6 +1145,17 @@ Format:
  ~~~~~ expression
 ~~~
 
+### Block nil argument (Prism only)
+
+Format:
+
+~~~
+(blocknilarg)
+"&nil"
+  ~~~ name
+ ~~~~ expression
+~~~
+
 ### Objective-C arguments
 
 MacRuby includes a few more syntactic "arguments" whose name becomes

lib/parser/meta.rb

@@ -7,7 +7,7 @@ module Meta
     # These are node types required by `Prism::Translation::Parser`, 
     # which has advanced syntax support ahead of the Parser gem.
     PRISM_TRANSLATION_PARSER_NODE_TYPES = %i(
-      itarg itblock
+      itarg itblock blocknilarg
     ).freeze
     private_constant :PRISM_TRANSLATION_PARSER_NODE_TYPES