Implement LSP/tsserver "signature help"

Both LSP and tsserver offer signature help commands for getting information about type signatures when writing functions. When turned on, an `autocmd` for detecting input would show information about the signature for the next argument of a function being typed, etc.

Start by reading ALE's existing LSP and tsserver command handling and locate the entry points for input-detection autocmds. The work is done when typing a function argument can trigger signature help from both LSP and tsserver, showing information for the next argument.

Written by the indexing model from the issue text.