fix: ignore extensions for directories when sorting

[sdht0]

Before this patch:

$ mkdir test1 test2 abc 01.city 02.apple
$ touch a.txt abc.txt abc.mp3 ab

$ eza                                       
01.city  02.apple  a.txt  ab  abc  abc.mp3  abc.txt  test1  test2

$ eza --sort=extension                      
ab  abc  test1  test2  02.apple  01.city  abc.mp3  a.txt  abc.txt

$ eza --sort=extension --group-directories-first
abc  test1  test2  02.apple  01.city  ab  abc.mp3  a.txt  abc.txt

Note how 01.city is sorted after 02.apple when sorting by extension.

After patch:

$ eza --sort=extension
01.city  02.apple  ab  abc  test1  test2  abc.mp3  a.txt  abc.txt

$ eza --sort=extension --group-directories-first
01.city  02.apple  abc  test1  test2  ab  abc.mp3  a.txt  abc.txt

Directories are treated exactly as if there are no . was found in the name.

Closes #821