chore: extract the first sequence of non-whitespace characters from the start of v:progname as progname

[nuffin]

Since I always start multiple vim instances without arguments, I want to know which directory each vim process is in, so I composed a shell function as:

vimc()
{
    /bin/bash -c 'exec -a "vim (in $(basename "$PWD"))" vim "$@"'
}

Then, the vim process will show as vim (in /path/to/blah/blah) in the output of the ps command. But the vim-delve plugin does not work with it because the cache directory is not found, so I fixed it as shown in this PR.