[FRONTEND] Add tl.expand_dims (#1614)

This exposes `semantic.expand_dims` in the public API and builds upon it
with support for expanding multiple dimensions at once. e.g.
```python
tl.expand_dims(tl.arange(0, N), (0, -1))  # shape = [1, N, 1]
```

Compared to indexing with `None`, this API is useful because the
dimensions can be constexpr values rather than hard-coded into the
source. As a basic example
```python
@triton.jit
def max_keepdim(value, dim):
    res = tl.max(value, dim)
    return tl.expand_dims(res, dim)
```

docs/python-api/triton.language.rst

@@ -34,6 +34,7 @@ Shape Manipulation Ops
     :nosignatures:
 
     broadcast_to
+    expand_dims
     reshape
     ravel