Merge #1138

1138: Add L_DOLLAR and R_DOLLAR r=matklad a=edwin0cheng

As discussion in issue https://github.com/rust-analyzer/rust-analyzer/issues/1132 and PR #1125 , this PR add 2 `Syntax::Kind` : `L_DOLLAR` and `R_DOLLAR` for representing `Delimiter::None` in mbe and proc_marco. By design, It should not affect the final syntax tree, and will be discard in `TreeSink`.

My original idea is handling these 2 tokens case by case, but i found that they will appear in every place in the parser (imagine `tt` matcher). So this PR only handle it in `Parser::do_bump` and `Parser::start`, although It will not fix the `expr` matcher executing order problem in original idea.


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

crates/ra_syntax/src/grammar.ron

@@ -118,6 +118,8 @@ Grammar(
         "LIFETIME",
         "COMMENT",
         "SHEBANG",
+        "L_DOLLAR",
+        "R_DOLLAR",
     ],
     nodes: [
         "SOURCE_FILE",