Reenable interactive operator completions

Disabled in #1247.

GHCi prior to version 8.0.1 have bug in `:complete` command: when completing operators it returns a list of all imported identifiers (see Track [ticket](https://ghc.haskell.org/trac/ghc/ticket/10576)). This leads to significant Emacs slowdown. To aviod slowdown operator completions are disabled for now.

Suggested solution:
1. In the beginning of a session issues a test `:complete >>=` command.
2. See if output is okay or not okay.
3. Mark GHCi session as one with a working `:complete` or a broken `:complete` command.
4. Do not complete operators in sessions with broken complete.

Start by locating the existing GHCi session initialization and handling for the `:complete` command. Reproduce `:complete >>=`, record how working and broken output differ, and make operator completion conditional on that session result while preserving the workaround for older GHCi versions.

Written by the indexing model from the issue text.