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/lib.rs

@@ -295,7 +295,8 @@ fn api_walkthrough() {
 
     // Let's get the `1 + 1` expression!
     let body: ast::BlockExpr = func.body().unwrap();
-    let expr: ast::Expr = body.tail_expr().unwrap();
+    let stmt_list: ast::StmtList = body.stmt_list().unwrap();
+    let expr: ast::Expr = stmt_list.tail_expr().unwrap();
 
     // Enums are used to group related ast nodes together, and can be used for
     // matching. However, because there are no public fields, it's possible to
@@ -331,8 +332,8 @@ fn api_walkthrough() {
     assert_eq!(text.to_string(), "1 + 1");
 
     // There's a bunch of traversal methods on `SyntaxNode`:
-    assert_eq!(expr_syntax.parent().as_ref(), Some(body.syntax()));
-    assert_eq!(body.syntax().first_child_or_token().map(|it| it.kind()), Some(T!['{']));
+    assert_eq!(expr_syntax.parent().as_ref(), Some(stmt_list.syntax()));
+    assert_eq!(stmt_list.syntax().first_child_or_token().map(|it| it.kind()), Some(T!['{']));
     assert_eq!(
         expr_syntax.next_sibling_or_token().map(|it| it.kind()),
         Some(SyntaxKind::WHITESPACE)