Please add edit_readonly function similar to edit_constant

#### My Pain

I'm trying to contribute a new Location widget to panel. Some of the parameters on this widget should be readonly but still be updated when the value of the corresponding Bokeh typescript model changes it value.

In order to enable this i need to set the parameters in code on the python side but this raises a `TypeError`.

If I change the parameters from `readonly` to `constant` then i can use the `param.edit_constant` contextmanager.

But the parameters should really be readonly.

#### Solution

Add a `param.edit_readonly` manager similar to the `param.edit_constant` manager.

Alternatively change `param.edit_constant` to a `param.edit_constant_or_readonly` manager.

#### Additional Context

For now I can just solve my problem in my own code by using an adjusted version of the below code.