Remove hard coded English text

[thomasdondorf]

Thank you for this great library!

Is your feature request related to a problem? Please describe.
There is currently hard coded English text in this library making it not possible to use it for other languages/i18n.

Describe the solution you'd like
Offer options to provide text instead of the hard coded "Multiple Panels" or just use a number without text (or no text?) by default:

dockview/packages/dockview-core/src/dnd/groupDragHandler.ts

Line 78 in d7ba69f

ghostElement.textContent = `Multiple Panels (${this.group.size})`;