Support arbitrary kwargs for materials (fixes #75)

[rainerrodrigues]

Description

This PR addresses issue #75 by providing a "fallback" dictionary for Material types, allowing users to pass arbitrary, unsupported Three.js properties directly to the browser.

Changes Made

• Added a kwargs::Dict{String, Any} = Dict{String, Any}() property to Texture, GenericMaterial, PointsMaterial, and FatLineMaterial in src/objects.jl.
• Updated the lower methods in src/lowering.jl to merge! the user's kwargs directly into the outgoing JSON dictionary right before it is returned.

Example Usage:
Users can now pass things like wireframe or wireframeLinewidth seamlessly:

using Colors, GeometryBasics
mat = MeshPhongMaterial(
    color=RGB(0., 0.5, 1.), 
    kwargs=Dict{String, Any}("wireframe" => true, "wireframeLinewidth" => 5)
)