[Minor] McStas version configuration registry to be read-only

[YooSunYoung]

The configuration per McStas version seems to be used in a few places so I turned it into a dataclass.
In this way, we don't have to always pass the default dictionary to make a new one.
We can simply add a new field with default value in the dataclass.

_get_mcstas_version_settings makes sure the registry is sorted before it tries to find the matching setting so now we don't have to worry about the registry being sorted.

I made both registry (mapping, dict) object and the dataclass to be read-only so that we can't accidentally mess it up.
In case you want to keep it editable, I added the keyword-only argument that you can pass a registry your self.

[mads-bertelsen]

Those updates make the structure neater and safer, thanks!