[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/__snapshots__/translation.test.ts.snap

@@ -10,8 +10,7 @@ exports[`translation context body of t-sets are translated in context 1`] = `
     ctx = Object.create(ctx);
     ctx[isBoundary] = 1
     setContextValue(ctx, \\"label\\", \`traduit\`);
-    const b2 = text(ctx['label']);
-    return multi([b2]);
+    return text(ctx['label']);
   }
 }"
 `;
@@ -94,6 +93,23 @@ exports[`translation context slot attrs and text contents are translated in cont
 }"
 `;
 
+exports[`translation context t-translation-context with several children 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div><div/><div/><block-child-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let b2;
+    if (true) {
+      b2 = text(\`\`);
+    }
+    return block1([], [b2]);
+  }
+}"
+`;
+
 exports[`translation context translation of attributes in context 1`] = `
 "function anonymous(app, bdom, helpers
 ) {
@@ -153,6 +169,21 @@ exports[`translation support body of t-sets inside translation=off are not trans
 }"
 `;
 
+exports[`translation support body of t-sets inside translation=off are not translated 2 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { isBoundary, withDefault, setContextValue } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    ctx[isBoundary] = 1
+    setContextValue(ctx, \\"label\\", \`untranslated\`);
+    return text(ctx['label']);
+  }
+}"
+`;
+
 exports[`translation support body of t-sets with html content are translated 1`] = `
 "function anonymous(app, bdom, helpers
 ) {
@@ -264,6 +295,23 @@ exports[`translation support t-set and falsy t-value: t-body are translated 1`]
 }"
 `;
 
+exports[`translation support t-translation with several children 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div><div/><div/><block-child-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let b2;
+    if (true) {
+      b2 = text(\`\`);
+    }
+    return block1([], [b2]);
+  }
+}"
+`;
+
 exports[`translation support translation is done on the trimmed text, with extra spaces readded after 1`] = `
 "function anonymous(app, bdom, helpers
 ) {