[IMP] app, compiler: introduce t-out

t-out automatically escaped content when it is a string not marked
with the `markup` function

t-out renders the raw content if it is a Block, or if it has been marked
with the `markup` funtion.

t-esc has been kept since it is safe and is optimized to render text nodes.

all t-raw calls are in fact the same as t-out.

src/utils.ts

@@ -21,3 +21,18 @@ export async function loadFile(url: string): Promise<string> {
   }
   return await result.text();
 }
+
+/*
+ * This class just transports the fact that a string is safe
+ * to be injected as HTML. Overriding a JS primitive is quite painful though
+ * so we need to redfine toString and valueOf.
+ */
+export class Markup extends String {}
+
+/*
+ * Marks a value as safe, that is, a value that can be injected as HTML directly.
+ * It should be used to wrap the value passed to a t-out directive to allow a raw rendering.
+ */
+export function markup(value: any) {
+  return new Markup(value);
+}