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 })

* reformat Lua runtime to make lint CI pass
* reduce max line length to 100

A alternative/subset of https://github.com/neovim/neovim/pull/18506  that should be forward compatible with a potential project system.

Configuration of LSP clients (without lspconfig) now looks like this:

    vim.lsp.start({
       name = 'my-server-name',
       cmd = {'name-of-language-server-executable'},
       root_dir = vim.fs.dirname(vim.fs.find({'setup.py', 'pyproject.toml'}, { upward = true })[1]),
    })

This is a pure Lua implementation of the Vim findfile() and finddir()
functions without the special syntax.

This function is modeled after the path.dir() function from Penlight and
the luafilesystem module.

vim.fs.parents() is a Lua iterator that returns the next parent
directory of the given file or directory on each iteration.