Modularize Searching: Write individual functions for checking usernames availability on each website

[GeekyShacklebolt]

Currently, usernames availability check is done in broader functions like check_on_sites_with_syntax_domain_username. We can modularize this checking in separate functions for each website and calling them in function check_username_availability. This might also be necessary because there would be different cases and approaches to check username availability on different websites. Therefore, keeping their checks modularize in separate functions would be good in later stages.

[GeekyShacklebolt]

Also, we need to validate usernames as per each website. For example, Twitter doesn't allow usernames longer than 15 characters, etc. These specific validations can be added to their individual username checking functions.