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/ast.rs

@@ -18,7 +18,7 @@ use crate::{
 };
 
 pub use self::{
-    expr_ext::{ArrayExprKind, Effect, ElseBranch, LiteralKind},
+    expr_ext::{ArrayExprKind, BlockModifier, ElseBranch, LiteralKind},
     generated::{nodes::*, tokens::*},
     node_ext::{
         AttrKind, FieldKind, Macro, NameLike, NameOrNameRef, PathSegmentKind, SelfParamKind,