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

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')