FR: add quick-fix to drop unused variables

[GitMensch]

similar to https://github.com/NEORIS-ZDEVOPS/CBL-Linter/tree/master/doc, obviously does not apply to elements in copybooks or variables which parent is used

possibly configurable to check for that (extension only based on the reference counter), maybe with two options (variables, procedures [or, to ease that: sections only [may still "fall through" but less likely - and possible to have GnuCOBOL enforcing it])