Hover provider for crossrefs

[gfontorbe]

Solves part of #355

Added hover behavior when hovering a cross-reference.

1. If the cross-reference references a type alternative, the hover content is the type alternative itself.

type T = A | B;

R : prop = [T];

Hovering over [T] displays type T = A | B;

2. If the cross-reference refers to a parser rule and has an explicit terminal.

R: prop = [T:X];

Hovering over [T:X] displays [T:X]

3. If the cross-reference refers to a parser rule and has no explicit terminal

R: prop = [T]

Hovering over [T] displays [T:X] with X being the terminal associated with the property name in the rule T

[msujew]

Looking mostly good, just some more minor issues.

[msujew]

Thanks, looks good to me!