Follow up to #4443

Since we reverted how it used to be (which I still think is the right default), some embedders/users may want it the old way still. We can solve this for everyone by adding a cursorInactiveStyle as suggested by @boettges in #4443 (comment). The name cursorInactiveStyle aligns with selectionInactiveBackground

To implement this you would need to:

1. Add the new option to xterm.d.ts, options service, etc.
2. Implement it for dom, canvas and webgl renderers