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/parser/src/syntax_kind/generated.rs

@@ -187,6 +187,7 @@ pub enum SyntaxKind {
     BREAK_EXPR,
     LABEL,
     BLOCK_EXPR,
+    STMT_LIST,
     RETURN_EXPR,
     YIELD_EXPR,
     MATCH_EXPR,
@@ -196,7 +197,6 @@ pub enum SyntaxKind {
     RECORD_EXPR,
     RECORD_EXPR_FIELD_LIST,
     RECORD_EXPR_FIELD,
-    EFFECT_EXPR,
     BOX_EXPR,
     CALL_EXPR,
     INDEX_EXPR,