Add find_linkable search in defenitions

[kashelkin]

I use a lot of definitions in my notes. And I think that as long as it is possible to make links to defenitions, maybe it could be useful to see them in Telescope find_linkable search results.

[max397574]

tbh I don't understand this regex but I'll just assume it still works and your feature works 😄

[kashelkin]

My regex change just adds matching for lines that start with $ or $$