Add "Anonymous only" to SpaceAfterFunctionNames

[dgxo]

I've always included a space after function with anonymous functions, it's just a habit now.

pcall(function ()
end)

task.spawn(function ()
end)

const empty = function () end

const function runThis()
end

But this isn't supported by StyLua - only all function definitions or none, and I think all definitions just looks odd.
It would be nice to have "Anonymous only" so a space is only added for anonymous functions.

I've looked through the code but I'm not too familiar with Rust and am unsure how you would want to implement this (in create_function_definition_trivia or a new function or something else) so I haven't submitted a PR.