[FEAT] Added string helper functions

[jaxwilko]

This PR adds some string helper methods for common indexing operations.

• index - adds the next index to a string
• unique - adds the next index to a string calculated from a list of strings
• uniqueFile - same as unique but working around the file extension to keep the index within the filename

Examples

'hello_world_1'       ===   Str::index('hello_world'));
'hello_world_2'       ===   Str::index('hello_world_1'));
'hello_world_43'      ===   Str::index('hello_world_42'));
'hello_world 3'       ===   Str::index('hello_world 2', separator: ' '));
'hello_world 6'       ===   Str::index('hello_world 2', separator: ' ', step: 4));
'hello_world8'        ===   Str::index('hello_world4', separator: '', step: 4));
'hello_world4_1'      ===   Str::index('hello_world4', step: 4));
'hello_world-22'      ===   Str::index('hello_world', separator: '-', starting: 22));
'winter cms 1'        ===   Str::index('winter cms', separator: ' '));
'winter cms 2'        ===   Str::index('winter cms 1', separator: ' '));

'winter_cms_4'        ===   Str::unique('winter_cms', ['winter_cms_1', 'test_5', 'winter_cms_3']));
'winter_cms 1'        ===   Str::unique('winter_cms', ['winter_cms_1', 'test_5', 'winter_cms_3'], ' '));
'winter_cms_1'        ===   Str::unique('winter_cms_1', ['winter_cms_1', 'test_5', 'winter_cms_3']));
'winter_cms_1'        ===   Str::unique('winter_cms', ['test_5']));

'winter_cms_4.test'   ===   Str::uniqueFile('winter_cms.test', ['winter_cms_1.test', 'test_5.egg', 'winter_cms_3.test']));
'winter_cms_1.test'   ===   Str::uniqueFile('winter_cms.test', ['test_5.test']));
'winter_cms_1.test'   ===   Str::uniqueFile('winter_cms_1.test', ['test_5.test']));