Make _qb.Path emit a shape with a splat by default

Closes #838

We want to make the query builder API have consistent behavior in
absence of explict query shapes.

Given these three cases:

```python
default.User

default.User.posts

default.User.select(
  posts=default.User.posts
)
```

... we'd want all paths without an explict `.select` return all pointers
of the target type, not just the id.

Current behavior:
- case 1: `select User { * }`,
- case 2: `select User.posts`,
- case 3: `select User { posts }`.

---

This PR changes that to:
- case 1: `select User { * }` (unchanged),
- case 2: `select User.posts { * }`,
- case 3: `select User { posts: { * } }`.

Author: aljazerzen

gel/_internal/_qb/_expressions.py

@@ -224,7 +224,9 @@ def __edgeql_expr__(self, *, ctx: ScopeContext) -> str:
 
         steps.append(edgeql(current, ctx=ctx))
 
-        return "".join(reversed(steps))
+        path_ql = "".join(reversed(steps))
+        shape_ql = " { * }" if self.is_link else ""
+        return path_ql + shape_ql
 
 
 @dataclass(kw_only=True, frozen=True)
@@ -822,7 +824,8 @@ def _render_shape(
                 and el_expr.source.type == source.type
                 and el_expr.name == el.name
             ):
-                el_text = _edgeql.quote_ident(el.name)
+                el_shape_text = " { * }" if el_expr.is_link else ""
+                el_text = _edgeql.quote_ident(el.name) + el_shape_text
             else:
                 assign = InfixOp(
                     lexpr=Ident(name=el.name, type_=el_expr.type),