fix(py): Only count first identifier in default_parameter (fix #148)

Default parameters such as:

```python
def test(foo=delimiter):
   pass
```

where counting `delimiter` as Parameters. To fix that, I only return the first child in a `default_parameter` tree

lua/neogen/configurations/python.lua

@@ -74,7 +74,7 @@ return {
                                         node_type = "default_parameter",
                                         subtree = {
                                             {
-                                                retrieve = "all",
+                                                position = 1,
                                                 node_type = "identifier",
                                                 extract = true,
                                                 as = i.Parameter,

lua/neogen/init.lua

@@ -299,7 +299,7 @@ end
 ---     with multiple annotation conventions.
 ---@tag neogen-changelog
 ---@toc_entry Changes in neogen plugin
-neogen.version = "2.15.0"
+neogen.version = "2.15.1"
 --minidoc_afterlines_end
 
 return neogen