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/ted.rs

@@ -162,8 +162,8 @@ fn ws_before(position: &Position, new: &SyntaxElement) -> Option<SyntaxToken> {
     }
 
     if prev.kind() == T!['{'] && ast::Stmt::can_cast(new.kind()) {
-        if let Some(block_expr) = prev.parent().and_then(ast::BlockExpr::cast) {
-            let mut indent = IndentLevel::from_element(&block_expr.syntax().clone().into());
+        if let Some(stmt_list) = prev.parent().and_then(ast::StmtList::cast) {
+            let mut indent = IndentLevel::from_element(&stmt_list.syntax().clone().into());
             indent.0 += 1;
             return Some(make::tokens::whitespace(&format!("\n{}", indent)));
         }