refactor: add has('wsl') to help set s:is_wsl

[xudyang1]

vim.fn.has() or has() can help check current runtime condition.

As :h has() indicates:

Returns 1 if {feature} is supported, 0 otherwise.
...
2. Runtime condition or other pseudo-feature. For example the
"win32" feature checks if the current system is Windows:
:if has("win32")
feature-list
List of supported pseudo-feature names:
... ......
wsl WSL (Windows Subsystem for Linux) system.

Inspired from:

https://github.com/neovim/neovim/blob/dc466f9a63ede5e2bbddc33688c0c972abfe9a52/runtime/autoload/provider/clipboard.vim#L132

[glacambre]

Thank you! :)