[IMP] template set config: getTemplate function

A template set can now obtain a template by calling the function
getTemplate (if any) received in the initial config.
The returned value can be an Element instance, a template string, a
function (i.e. a compiled template), or null. In the last case, owl will
look into the template set rawTemplates object.

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

@@ -101,3 +101,55 @@ exports[`loading templates can load a few templates from an XMLDocument 2`] = `
   }
 }"
 `;
+
+exports[`loading templates getTemplate: element returned (2) 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div>Hello World!</div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    return block1();
+  }
+}"
+`;
+
+exports[`loading templates getTemplate: element returned 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div>Hello World!</div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    return block1();
+  }
+}"
+`;
+
+exports[`loading templates getTemplate: template string returned 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div>Hello World!</div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    return block1();
+  }
+}"
+`;
+
+exports[`loading templates getTemplate: undefined returned 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div>Hello World!</div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    return block1();
+  }
+}"
+`;