Use mouse position for diagnostic and fold preview

[PgBiel]

Fixes #73

Now diagnostic and fold preview show information from under the mouse on hover, not under the cursor.

I haven't changed the other providers (which also have this bug) since they depend on the cursor to get information (i.e. <cword>). Those could require a larger refactor.

More generally, all providers seem to only work with the current window, whereas, in a window split, the current window might not be the window the mouse is hovering on, returning wrong information. Fixing this would be best done in a separate PR as the changes would be quite large.

So, for this PR, I try to bring the most value for the smallest changes. Let me know if any further changes are needed.

[lewis6991]

I assume the cursor position is still used if the key mapping is used instead?

[PgBiel]

Is that information passed to opts? Because if so, execute would work but I'd have to change enabled a bit (shouldn't be too difficult).

[PgBiel]

I assume the cursor position is still used if the key mapping is used instead?

Okay! I managed to implement this by using opts for enabled. Tested the keybind and the mouse versions and they both seem to report information for the correct positions, for both diagnostic and fold preview providers. 👍

[PgBiel]

I also managed to fix this from the PR description:

More generally, all providers seem to only work with the current window, whereas, in a window split, the current window might not be the window the mouse is hovering on, returning wrong information. Fixing this would be best done in a separate PR as the changes would be quite large.

Actually, by using the bufnr provided through opts it works fine with window splits... so I just had to change fold preview to use bufnr through a buf_call, much simpler than I thought. Yay!

Fixing the other providers could require some additional work due to <cword>, however, so that's best kept to a separate PR.