Merge #2628

2628: Add macro 2.0 support in parser r=matklad a=edwin0cheng

This PR added a new syntax kind : `MACRO_DEF` and a keyword `MACRO_KW`

there are two syntax for declarative macro 2.0 :
1. Normal : `macro m { ($i:ident) => {} }` , which handle similar to legacy one. 
2. Call like: `macro m($i:ident) {}`, it produces a single token tree which have two child token trees : `($i:ident)` and `{}`

Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>

crates/ra_syntax/src/grammar.ron

@@ -94,7 +94,8 @@ Grammar(
         "return",
         "try",
         "box",
-        "await"
+        "await",
+        "macro"
     ],
     contextual_keywords: [
         "auto",
@@ -140,6 +141,7 @@ Grammar(
         "TYPE_ALIAS_DEF",
         "MACRO_CALL",
         "TOKEN_TREE",
+        "MACRO_DEF",
 
         "PAREN_TYPE",
         "TUPLE_TYPE",