Draft: Added `:find[!]` ghci command

* `:find  <name>` - find a term / expression identified by <name> in any
                   available modules
* `:find! <name>` - find a term / expression which identifier incluedes
                    <name> as a substring.

Please take a few moments to verify that your commits fulfill the following:

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• I expect :find to be quite friendly for both fresh and experienced haskellers.

If you have any questions don't hesitate to open your merge request and inquire in a comment. If your patch isn't quite done yet please do add prefix your MR title with WIP:.