feat: autofix quote between __argc_filter and _argc_prefix

[sigoden]

when you completes key-value seperated by =, you write:

_choice_fn() {
    echo "__argc_prefix:${ARGC_FILTER%%=*}="
    echo "__argc_filter:${ARGC_FILTER#*=}"
    echo foo
    echo bar
    :;
}

But if you type foo=', no completion exists becasuse filter is '.
this pr will automaticlly move start quote from __argc_filter to_argc_prefix, make things easier