Improve the number formatting with input validation and customizable separators

[Kritika75]

Hey @UTSAVS26 I made the following changes in number formatting and test number formatting files:

1. Added input validation to check if the input is an integer or a float, raising a ValueError if not
2. Introduced parameters thousands_sep and decimal_sep, allowing customization of thousands and decimal separators.
3. Added logic to replace the default thousands and decimal separators with user-defined ones, ensuring that the function can format numbers according to different localization standards and included the additional examples

And for test file:

1. Added a test case to check if the function correctly formats numbers with custom thousands and decimal separators.
2. Added a test to ensure that the function raises a ValueError when the input is invalid (not an integer or float).
3. Proper formatting of integers and floats and handling of negative numbers.

Issue number #29

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[Kritika75]

Also @UTSAVS26 if everything is fine please provide the labels to my pr, thank you