Support non-default script path

[lucaswerkmeister]

See maxlath/wikibase-sdk#77 for rationale. I assumed the hard-coded /w/ would mostly be in that library, but it looks like -edit also has it:

wikibase-edit/lib/request/post.js

Line 40 in d42636f

url: buildUrl(`${instance}/w/api.php`, query),

wikibase-edit/lib/parse_instance.js

Lines 12 to 16 in d42636f

	config.instance = instance
	.replace(/\/$/, '')
	.replace('/w/api.php', '')
	config.instanceApiEndpoint = `${config.instance}/w/api.php`

wikibase-edit/lib/resolve_title.js

Line 19 in d42636f

const infoUrl = `${instance}/w/api.php?action=query&meta=siteinfo&siprop=namespaces&format=json`