internal: more reasonable grammar for blocks

Consider these expples

        { 92 }
  async { 92 }
    'a: { 92 }
   #[a] { 92 }

Previously the tree for them were

  BLOCK_EXPR
    { ... }

  EFFECT_EXPR
    async
    BLOCK_EXPR
      { ... }

  EFFECT_EXPR
    'a:
    BLOCK_EXPR
      { ... }

  BLOCK_EXPR
    #[a]
    { ... }

As you see, it gets progressively worse :) The last two items are
especially odd. The last one even violates the balanced curleys
invariant we have (#10357) The new approach is to say that the stuff in
`{}` is stmt_list, and the block is stmt_list + optional modifiers

  BLOCK_EXPR
    STMT_LIST
      { ... }

  BLOCK_EXPR
    async
    STMT_LIST
      { ... }

  BLOCK_EXPR
    'a:
    STMT_LIST
      { ... }

  BLOCK_EXPR
    #[a]
    STMT_LIST
      { ... }

crates/syntax/src/tests/ast_src.rs

@@ -149,6 +149,7 @@ pub(crate) const KINDS_SRC: KindsSrc = KindsSrc {
         "BREAK_EXPR",
         "LABEL",
         "BLOCK_EXPR",
+        "STMT_LIST",
         "RETURN_EXPR",
         "YIELD_EXPR",
         "MATCH_EXPR",
@@ -158,7 +159,6 @@ pub(crate) const KINDS_SRC: KindsSrc = KindsSrc {
         "RECORD_EXPR",
         "RECORD_EXPR_FIELD_LIST",
         "RECORD_EXPR_FIELD",
-        "EFFECT_EXPR",
         "BOX_EXPR",
         // postfix
         "CALL_EXPR",