Make "non-standard" keywords in methods keyword-only arguments

[hoxbro]

Some methods in Lumen need different inputs depending on the class it is called from. An example of this could be from_spec, the example below. I would like to see these non-standard keywords being keyword-only arguments, so passing in the wrong arguments is impossible.

https://github.com/holoviz/lumen/blob/95809959b18eb31d069c084f4fabdefbc2cfb05e/lumen/views/base.py#L267-L269

https://github.com/holoviz/lumen/blob/95809959b18eb31d069c084f4fabdefbc2cfb05e/lumen/dashboard.py#L299