Add a way to filter the location of template parts within themes & child themes

Currently, within a theme / child theme, we're limited to grabbing things from `template-parts/components/`, and call the template parts _must_ be prefixed with `component-`. This is fine, but in the context of a theme, might not make the most sense. Having the ability to filter these would be aces. 👌

Start by locating the code that resolves template parts under template-parts/components/ and applies the component- filename prefix for themes and child themes. Trace how a theme-specific location is selected, then define and verify a filterable location and naming behavior for both theme types.

Written by the indexing model from the issue text.