Add to_snake_case and to_camel_case for index label conversion using …
**Add `to_snake_case` and `to_camel_case` methods for Index label conversion**

This PR adds two new string transformation methods to the `pandas.Index` class:

### 🚀 New methods
- `to_snake_case()`: Converts string index labels to `snake_case` using `inflection.underscore`
- `to_camel_case()`: Converts string index labels to `camelCase` using `inflection.camelize`

Both methods:
- Leave non-string values (e.g. integers, `None`) unchanged
- Are chainable and return a new `Index` instance

### 🧪 Tests
Added unit tests for both methods in `test_base.py`:
- Covers strings with mixed case and spaces
- Verifies behavior with mixed-type labels

### 📌 Example
```python
import pandas as pd

df = pd.DataFrame({"first name": [1], "another_column": [1]})
df.columns = df.columns.to_camel_case()
print(df.columns)
# Index(['firstName', 'anotherColumn'], dtype='object')

df.columns = df.columns.to_snake_case()
print(df.columns)
# Index(['first_name', 'another_column'], dtype='object')
```


Thanks for the PR, but it appears this feature is not tied to an open issue with core developer support. For new APIs, we would need core developer support before adding new API. (Note given the simplicity of this feature, it's unlikely that this would be added to pandas.)

Thanks for the suggestion but closing  Thanks for your comment and understand your point. This is actually something I've always wanted when dealing with messy column names. Without modifying pandas itself, the code below is the alternative I can think of. Do you have any recommendations?

```
import pandas as pd
import inflection

@pd.api.extensions.register_index_accessor("clean")
class ColumnIndexAccessor:
    def __init__(self, pandas_obj):
        self._obj = pandas_obj

    def to_snake_case(self):
        return self._obj.to_series().apply(
            lambda x: inflection.underscore(x).replace(" ", "_")
        ).values

    def to_camel_case(self):
        return self._obj.to_series().apply(
            lambda x: inflection.camelize(x, uppercase_first_letter=False)
        ).values
```
