[REF] code_generator: move generating code to CodeTarget

Before this commit, we had two places with code that generate a function
code. Now, all this code is moved in a method 'generateCode' on
CodeTarget.

tests/compiler/__snapshots__/t_foreach.test.ts.snap

@@ -493,7 +493,6 @@ exports[`t-foreach with t-memo 1`] = `
 ) {
   let { text, createBlock, list, multi, html, toggler, component } = bdom;
   let { withDefault, getTemplate, prepareList, withKey, zero, call, callSlot, capture, isBoundary, shallowEqual, setContextValue, toNumber, safeOutput } = helpers;
-  let assign = Object.assign;
   
   let block1 = createBlock(\`<div><block-child-0/></div>\`);
   let block3 = createBlock(\`<p><block-text-0/><block-text-1/></p>\`);
@@ -518,7 +517,7 @@ exports[`t-foreach with t-memo 1`] = `
       let d1 = ctx['item'].x;
       let d2 = ctx['item'].y;
       c_block2[i1] = withKey(block3([d1, d2]), key1);
-      nextCache[key1] = assign(c_block2[i1], {memo: memo1});
+      nextCache[key1] = Object.assign(c_block2[i1], {memo: memo1});
     }
     let b2 = list(c_block2);
     return block1([], [b2]);