feat: Allow path separator customization

[scolby33]

Description

This is based on #5030, just updated against current master and resolved conflicts. The following description is mostly copied from that PR description.

Adds a new option separator_symbol to the directory module, that allows to define a replacement for the default/OS path separator.
The option is set to an empty string by default, which basically deactivates it and makes it backward compatible to the current behavior. If set to an non empty string, it will overrule the option use_os_path_sep and use the value everywhere the OS specific separator would have been used.

Motivation and Context

Allows to further customize the directory module, e. g. to achieve powerline style paths.

Closes #1222
Supersedes #5030

Screenshots (if appropriate):

https://user-images.githubusercontent.com/4013111/227817246-d2d1ab42-a7a2-4ce5-a6c3-7fcfd66741d9.png

How Has This Been Tested?

• I have tested using MacOS
• I have tested using Linux
• I have tested using Windows

Tests included running unit tests with cargo test and executing a debug build with a customized configuration.

Checklist:

• I have updated the documentation accordingly.
• I have updated the tests accordingly.