Add special "instance or subclass" mode

[vostrnad]

Fixes #13

When the traversal property is set to "instance of", a checkbox appears that changes the behavior from traversing the property "instance of" (which is rarely useful) to traversing superclasses of direct superclasses in the forward direction, and traversing subclasses to direct instances in the reverse direction (instances are connected with dashed lines to distinguish them from subclasses). Bidirectional mode is supported, though undirected isn't–right now the graph simply crashes–and neither are maximum iterations and the size property. I've also made some pretty invasive changes to the code to make this feature easier to implement, so all in all I don't think this should be merged in the current state, but I'm sharing it for anyone who might find it useful, and I'd be more than happy to implement suggestions that would make this fit for merging.

[AngryLoki]

Hi, thank you for your pull-request!
This change looks additive and does not break other flows. I think I can merge it.
What it is currently missing is some good example in https://github.com/AngryLoki/wikidata-graph-builder/blob/master/src/routes/About.svelte - could you add it?

Regarding overall approach, I think somewhat better query is

SELECT DISTINCT ?item ?itemLabel ?linkTo ?linkType {
  {
    ?item wdt:P279* wd:Q169470 .
    OPTIONAL { ?item wdt:P279 ?linkTo }
  }
  UNION
  {
    ?class wdt:P279* wd:Q169470 .
    ?item wdt:P31 ?class .
    OPTIONAL { ?item wdt:P31 ?linkTo . BIND("alt" AS ?linkType) }
  }
  SERVICE wikibase:label {bd:serviceParam wikibase:language "en" }
}

i. e. output pairs into ?item ?linkTo and connection type to ?linkType. This approach is common in graph formats (maps to Gephi, graphviz, etc.).

[vostrnad]

I changed the query as per your suggestion and added two examples.