feat: Allow get_cwd() function to return an array

[riton]

This P.R allows get_cwd() function to return an array.

My motivation is to use a custom get_cwd() function such as

get_cwd = function(params)
  return {
    vim.fn.expand(('#%d:p:h'):format(params.context.bufnr)), -- default value
    vim.fn.getcwd(),
  }
end

that allows to have completion for both the buffer cwd (current behavior) and for the cwd where neovim was opened for relative paths.

I've tried to handle the get_cwd() returns a string case not to break user configurations with custom get_cwd(). So, no backward incompatibility is expected.

:warning: I'm a real beginner in lua so the code may be far from optimal :student:

[zhou13]

I have the same problem. The default relative path does not work sometimes. It is better if your example cwd is the default behavior. Thank you for the PR!