Previously, we used rustyline's filename completer. This allowed us to make
progress on the completion engine without building all the parts at once. We now
need our own filename completer to make progress.
The primary driver to having our own filename completer is that it can better
integrate with our path constructs. For example, if we have
> ls .../<TAB>
we want to show a list of suggestions that includes all files two directories up
from the current working directory. The least jarring experience to a user would
be to maintain the three dots. The easiest way for us to do this is by building
our own completer and path constructs.
The completion engine maps completion locations to spans on a line, which
indicate whther to complete a command name, flag name, argument, and so on.
Initial implementation is simplistic, with some rough edges, since it relies
heavily on the parser's interpretation. For example
du -
if asking for completions, `-` is considered a positional argument by the
parser, but the user is likely looking for a flag. These scenarios will be
addressed in a series of progressive enhancements to the engine.
