Allow passing kwargs to Electron window (fixes #122)

[rainerrodrigues]

Description

This PR allows users to pass keyword arguments through open(vis) down to the Electron window instantiation, resolving issue #122.

Changes Made

• Added ; kwargs... to Base.open(core::MeshCat.CoreVisualizer, w::Electron.Application) in ext/ElectronExt.jl.
• Forwarded kwargs... into Electron.Window().
• The main Base.open method in src/servers.jl was already configured to forward kw..., so this change seamlessly completes the pipeline.

Example Usage:

using MeshCat, Electron
vis = Visualizer()
app = Electron.Application()
open(vis, app; options=Dict("width" => 1920, "height" => 1080))