Issue 6807 listbin

[phadej]

Partially resolves #6807

I need to test this on Windows and with GHCJS

[phadej]

This also fixes Windows issue where plan.json didn't have executable extension:

PS C:\Users\Oleg Grenrus\Documents\cabal-plan> cabal run cabal-plan list-bins
Linking C:\Users\Oleg Grenrus\Documents\cabal-plan\dist-newstyle\build\x86_64-windows\ghc-8.6.5\cabal-plan-0.7.0.0\x\cabal-plan\build\cabal-plan\cabal-plan.exe ...
cabal-plan:exe:cabal-plan  C:\Users\Oleg Grenrus\Documents\cabal-plan\dist-newstyle\build\x86_64-windows\ghc-8.6.5\cabal-plan-0.7.0.0\x\cabal-plan\build\cabal-plan\cabal-plan

PS C:\Users\Oleg Grenrus\Documents\cabal-plan> & '...\cabal-install-3.3.0.0\x\cabal\build\cabal\cabal.exe' run cabal-plan list-bins
Up to date
cabal-plan:exe:cabal-plan  C:\Users\Oleg Grenrus\Documents\cabal-plan\dist-newstyle\build\x86_64-windows\ghc-8.6.5\cabal-plan-0.7.0.0\x\cabal-plan\build\cabal-plan\cabal-plan.exe

But AFAICS it will also add jsexe to GHCJS executables, which may or may not break some scripts when cabal-install is updated to 3.4. Missing .jsexe was a bug, not a feature.