Make filter subpath optional

Problem/Motivation

In a test transition from facet blocks + facets pretty paths → to this module atop the new Facets 3.x facets exposed filters system, I found I could almost achieve the same pathing, but would need to eliminate this module's "filter subpath" path part (which I believe is "filter" by default).

So instead of this:
/view-path/filter/exposed-filter/value

I would end up with this:
/view-path/exposed-filter/value

Steps to reproduce

N/A.

Proposed resolution

1. Determine if this is even possible. I suspect it is.

2. Make filter subpath optional. Perhaps add a checkbox saying "Enable filter subpath" that is checked by default, and if checked, present the "Filter subpath" text field to collect a custom filter subpath.

3. Adjust request logic to work with whatever is configured (with or without a filter subpath).

Remaining tasks

MR + review.

User interface changes

Filter subpath becomes optional.

API changes

None.

Data model changes

None.