New regex for Botswana numbers

Phone number information was found from the following sources:
- https://www.howtocallabroad.com/botswana/
- https://countrycode.org/botswana

Included the new case for Botswana (en-BW)

Added some test cases to check length for the phone number & area code pattern.

Fixed the pattern for Botswana mobile phone numbers and edited the test cases. I followed the convention from Table 8 of the national numbering plan document and the mobile number pattern on the Wiki page.

Oops! Looks like the file didn't update with some of my modified test cases - my apologies!