Make `offset` work in entities default config

[FrnchFrgg]

The way offset is fetched, it ignores any value set in the default.entities object. But having the same offset on every entity does not give the same result than using a global offset, and is thus worthwhile to have.

[dbuezas]

Again?
Oh yes, it shouldn't be entityIn: https://github.com/dbuezas/lovelace-plotly-graph-card/blob/master/src/plotly-graph-card.ts#L409

I keep making different variations of the same mistake.

Note to self: put the defaults merging logic in a different function and hide the unmerged entity variable from the scope after merging