Add number of references in hover of variables

[engboris]

Closes #354

The formatting could be improved (it is naive) and is not well displayed for groups.

[engboris]

Unfortunately, it's failing in some cases with a Request textDocument/hover failed. popup on VSCode.

For instance, when hovering EXPECT-ORDER in:

* IF   EXPECT-ORDER  =  'ASCII'
*    IF G NOT = "12345abcde"
* ELIF EXPECT-ORDER  =  'EBCDIC'

or P050-SEARCH, P050-00, +AM005* or even special case 1 in:

       program-id. test.
       procedure division.
      *---------------------------------------------------------------*
       P050-SEARCH SECTION.
       P050-00.
+AM005*    special case 1
+AM005     continue.
+AM005*
+AM005 P050-40.
           continue.
      *
       P050-99.
           EXIT.
      /

And more generally, the error occurs for every token having an associated reference codelens.