Populate positional vars for non-RHS holes

[isovector]

That is, if i write

case foo of
  Bar a b c -> _

the hole should get (foo.[[a,b,c]]) inserted in its position vars judgement

[isovector]

haskell-language-server/plugins/tactics/src/Ide/Plugin/Tactic.hs

Lines 335 to 346 in 8e69004

	getRhsPosVals :: RealSrcSpan -> TypecheckedSource -> Maybe (OccName, [OccName])
	getRhsPosVals rss tcs = getFirst $ everything (<>) (mkQ mempty $ \case
	TopLevelRHS name ps
	(L (RealSrcSpan span) -- body with no guards and a single defn
	(HsVar _ (L _ hole)))
	| containsSpan rss span -- which contains our span
	, isHole $ occName hole -- and the span is a hole
	-> First $ do
	patnames <- traverse getPatName ps
	pure (occName name, patnames)
	_ -> mempty
	) tcs

implements this for function RHSes. The more general solution is likely to just extend this.