[FIX] slots: better heuristic to determine named slot content

Unfortunately, we chose to use the directive `t-set` to define sub slot
contents in a template.

The goal was to reuse a directive for a similar use case: defining sub
template is almost the same as defining a slot content.

Obviously, this introduces a name conflict: the inner content of a
component cannot use t-set t-value anymore (nor t-set with a body
value), since they are interpreted as slot names.

We mitigate the issue here by only interpreting as slot content the
`t-set` statement located immediately below the parent component tag
name and with a body content.

However, a real fix need to introduce an additional directive to resolve
the ambiguity.

tests/component/__snapshots__/slots.test.ts.snap

@@ -469,6 +469,21 @@ exports[`t-slot directive slots are rendered with proper context, part 4 2`] = `
 }"
 `;
 
+exports[`t-slot directive t-set t-value in a slot 1`] = `
+"function anonymous(context, extra
+) {
+    // Template name: \\"__template__1\\"
+    let h = this.h;
+    let c4 = [], p4 = {key:4};
+    let vn4 = h('span', p4, c4);
+    const slot5 = this.constructor.slots[context.__owl__.slotId + '_' + 'default'];
+    if (slot5) {
+        slot5.call(this, context.__owl__.scope, Object.assign({}, extra, {parentNode: c4, parent: extra.parent || context}));
+    }
+    return vn4;
+}"
+`;
+
 exports[`t-slot directive template can just return a slot 1`] = `
 "function anonymous(context, extra
 ) {