[FIX] parser, code_generator: wrapped children recognition

Several directives (t-key, t-log, t-translation, …) are represented by
wrapper ASTs that contain another AST or null.
Because these wrappers don't share the same type as their children,
various AST type checks were broken.

This commit addresses those issues by:

- Commuting Translation / TranslationContext and Multi ASTs so that
  Multi children are spread as expected (see parseChildren).

- Parsing the t-key directive before t-esc / t-out (in line with https://github.com/odoo/owl/pull/1685).

- Ensuring wrappers around TSet ASTs are recognized as having no
  representation, so compileMulti properly discards children equivalent
  to TSet.

tests/compiler/t_key.test.ts

@@ -63,4 +63,10 @@ describe("t-key", () => {
 
     expect(renderToString(template2, { key: "1" })).toBe("<div><h1></h1></div>");
   });
+
+  test("t-key: interaction with t-esc", async () => {
+    const template = `<p t-key="key" t-esc="text"/>`;
+
+    expect(renderToString(template, { key: "1", text: "abc" })).toBe("<p>abc</p>");
+  });
 });