feat: allow multiline arg passing

[antoinemadec]

By reusing get_trigger_characters(), we can now trigger signature completion on multiline.
Only difference with before is that it adds a blank space in the list of trigger characters.

Useful when doing stuff like:

my_func(
    arg0,
    arg1,
    ...
)

Only drawback is that we will make signatureHelp LSP requests a bit more often I guess?