Add filtering to path_autocomplete

We recently committed https://github.com/backdrop/backdrop-issues/issues/3764.

But belated request/idea: working on contexts PR and I want to use this to autocomplete, but filtered by entity type. Currently once you start typing it shows you every piece of content/term/whatever.

But what if we changed the path to path-autocomplete/% and passed the type of content to the path_autocomplete($string = '', $type = NULL) function to filter by that type? Or even path-autocomplete/%/% and path_autocomplete($string = '', $type = NULL, $exclude = FALSE) to allow a negative filter.

In a form you could set '#autocomplete_path' => 'path-autocomplete/node to get only nodes, or, as in the case I'd like to accomplish, '#autocomplete_path' => 'path-autocomplete/' . $context->type.