Bumpped minimum vimeo/psalm version to 3.15

Enabled 'ordered_imports' in php-cs-fixer config

Updated README header design

Additional README updates

Fixed substring docs link

Replaced aliases with more relevant test cases

Added Str::camelCase(), Str::studlyCase(), Str::pascalCase, Str::snak…

…eCase() and Str::kebabCase() methods

Updated and restructured ConfigTest

README and docblock updates for Str::encrypt() and Str::decrypt() met…

…hods

Updated PHP requirement in the README