This patch implements :prompt completion.
The completion mechanism only provides completions when there is at least
one argument specified (prompt text).
The mechanism is based on other commands' completions and works as follows:
1. Attempts to look up a command by the name specified in args[1].
2.a On success it uses command.Complete.
2.b Otherwise, if total arguments count is lesser or equals than 2
(i.e. no command arguments specified), it attempts to complete
the command's name.
Additional effort is made to preserve prompt text, which often contains
spaces and formatting.
Signed-off-by: Sergey Smirnykh <sergey.smirnykh@siborgium.xyz>
Acked-by: Robin Jarry <robin@jarry.cc>
Usage:
:prompt <prompt> <command...>
Displays the prompt on the status bar, waits for user input, then
appends that input as the last argument to the command and executes it.
The input is passed as one argument to the command, unless it is empty,
in which case no extra argument is added.
