hover/goto should show something useful

[leecannon]

Currently hover/goto (ctrl+hover in vscode) shows the first token in the declaration.

For example with std.builtin.AtomicOrder, goto hover over std displays pub and over builtin displays const as those keywords just happen to be at the beginning of std.zig and builtin.zig.

I don't know exactly what we should show there but the current behaviour is at least vaguely useful for types and functions: