HoverContext should have non-optional readonly members

[jrieken]

Testing #210904

Because it is VS Code passing a HoverContext instance we should mark its properties readonly and unless I am mistaken there is no reason the declare them as optional. AFAIK when having a context (which is optional) all properties are defined

[aiday-mar]

We thought that it may be possible we add more fields into the hover context not in relation with the verbosity, hence why we made the fields optional.

We could perhaps add an optional sub-field into the HoverContext called HoverVerbosityRequest or HoverVerbosityContext with obligatory verbosity specific fields:

interface HoverContext {
    hoverVerbosityRequest?: HoverVerbosityRequest;
}

interface HoverVerbosityRequest {
    previousHover: Hover;
    action: HoverVerbosityAction;
}

Or we could keep the fields directly inside of the HoverContext but rename it to HoverVerbosityContext instead.