[IMP] qweb: turn handlers into function expressions only

For the sake of consistency with vanilla JS, and to allow some things
that were previously not possible.

tests/components/__snapshots__/higher_order_component.test.ts.snap

@@ -130,7 +130,8 @@ exports[`basics sub widget is interactive 1`] = `
   let block1 = createBlock(\`<span><button block-handler-0=\\"click\\">click</button>child<block-text-1/></span>\`);
   
   return function template(ctx, node, key = \\"\\") {
-    let d1 = [ctx, 'inc'];
+    const v1 = ctx['inc'];
+    let d1 = [v1, ctx];
     let d2 = ctx['state'].val;
     return block1([d1, d2]);
   }