Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.
Makes it possible to use `vim.fs.find` to find files where only a
substring is known.
This is useful for `vim.lsp.start` to get the `root_dir` for languages
where the project-file is only known by its extension, not by the full
name.
For example in .NET projects there is usually a `<projectname>.csproj`
file in the project root.
Example:
vim.fs.find(function(x) return vim.endswith(x, '.csproj') end, { upward = true })
