fix: backward slash in url when using Windows

[AnsonH]

Bug

I'm using Neovim on Windows. I'm trying to run :OpenInGHFile inside a file that's inside a nested directory from the repo's root:

lvim/       (https://github.com/AnsonH/lvim)
├── lua/
│   └── user/
│       └── plugins.lua    <-- In this file
└── config.lua

However, the URL contains encoded backward slashes (%5C) and returns a "File not found" page:

https://github.com/AnsonH/lvim/blob/main/lua%5Cuser%5Cplugins.lua

Reason

It's caused by inconsistent file separator (/ vs \) in util.get_current_relative_file_path:

-- `nvim_buf_get_name` returns a path with backward slashes on Windows
-- e.g. C:\Users\Anson\AppData\Local\lvim\lua\user\plugins.lua
local absolute_file_path = vim.api.nvim_buf_get_name(0)

-- However this always return a path with forward slashes
-- e.g. C:/Users/Anson/AppData/Local/lvim
local git_path = vim.fn.system("git rev-parse --show-toplevel")