Use abbreviation "expr" instead of "expression" (#3375)

Part of switching to the [abbreviations we've decided to
use](https://docs.google.com/document/d/1RRYMm42osyqhI2LyjrjockYCutQ5dOf8Abu50kTrkX0/edit?resourcekey=0-kHyqOESbOHmzZphUbtLrTw#heading=h.pph7i5m5un7q).

I will rename files in a follow-up PR.

toolchain/parse/handle_statement.cpp

@@ -47,8 +47,8 @@ auto HandleStatement(Context& context) -> void {
       break;
     }
     default: {
-      context.PushState(State::ExpressionStatementFinish);
-      context.PushStateForExpression(PrecedenceGroup::ForExpressionStatement());
+      context.PushState(State::ExprStatementFinish);
+      context.PushStateForExpr(PrecedenceGroup::ForExprStatement());
       break;
     }
   }
@@ -116,7 +116,7 @@ auto HandleStatementForHeaderIn(Context& context) -> void {
 
   state.state = State::StatementForHeaderFinish;
   context.PushState(state);
-  context.PushState(State::Expression);
+  context.PushState(State::Expr);
 }
 
 auto HandleStatementForHeaderFinish(Context& context) -> void {
@@ -180,7 +180,7 @@ auto HandleStatementReturn(Context& context) -> void {
 
   context.AddLeafNode(NodeKind::ReturnStatementStart, context.Consume());
   if (!context.PositionIs(Lex::TokenKind::Semi)) {
-    context.PushState(State::Expression);
+    context.PushState(State::Expr);
   }
 }