fix: remove extra trailing slash in filepath.expand

[levtuxov]

No description provided.

[lpil]

Hello! I'm not sure what this is because there's no issue, description, or tests.

Could you write tests and update the changelog please 🙏

[levtuxov]

Hi!

PS> dir env:\OS

Name                           Value
----                           -----
OS                             Windows_NT


PS> gcim win32_operatingsystem

SystemDirectory     Organization BuildNumber RegisteredUser SerialNumber            Version   
---------------     ------------ ----------- -------------- ------------            -------   
C:\Windows\system32              26100       Op             00436-20000-00000-AAOEM 10.0.26100


PS> G:\gleam_scratch\filepath git:[windows]
gleam test
  Compiling filepath
   Compiled in 3.21s
    Running filepath_test.main
........F.
Failures:

  1) filepath_test.expand_test
     Values were not equal
     expected: Ok("/one")
          got: Ok("/one/")
     output: 

Finished in 0.201 seconds
10 tests, 1 failures 

The PR fixes this error (on Windows)

[lpil]

Ah! Thank you!