[FIX] compiler: throw if using t-slot on a htmlelement tag

Before this commit, the tagName of a `t-slot` directive was ignored. So,
<t t-slot="owl">...</t>  and <div t-slot="owl">...</div> were basically
equivalent.

This is not really acceptable, since it most likely hides a developer
error. So, the question is what to do about it? There are two obvious
solutions:

- either make the `t-slot` directive behave as t-esc: the above example
would then be equivalent to <div><t t-slot="owl">...</t></div>
- or throw an error.

However, the first solution seems to be ambiguous when using a component
with slot props. How should the following code be interpreted?
   <SomeComponent a="a" b="b" t-slot="owl"/>
Should the 'a' and 'b' props be considered props for the component or
for the slot?

I guess we could make an exception for components, but it seems more
complicated than what it should be. Because of that, it seems simpler to
just throw an error.

closes #1354

tests/compiler/parser.test.ts

@@ -1686,6 +1686,12 @@ describe("qweb parser", () => {
     });
   });
 
+  test("t-slot on a div tag should throw", async () => {
+    expect(() => parse(`<div t-slot="name"/>`)).toThrowError(
+      "Directive 't-slot' can only be used on <t> nodes (used on a <div>)"
+    );
+  });
+
   // ---------------------------------------------------------------------------
   // t-debug
   // ---------------------------------------------------------------------------