Allow feature/varialble name to be next to )

[maxim-h]

I've always had problems when using some more involved patterns with subset.Seurat, e.g.:

> subset(seurat, subset = !grepl("cycling", celltype))
Error in `FetchData()`:
! None of the requested variables were found: 
Run `rlang::last_trace()` to see where the error occurred.

Turns out the WhichCells strips the opening bracket, but not the closing one, when searching for feature/variable names in expression.

I've fixed the issue for myself with this small change, but not sure if this was on purpose and my change breaks something else.

[maxim-h]

Actually, after taking another look I've found the base R function that seems to do exactly what is needed here without the manual tinkering with strings. Hence the second commit.

Maybe that's a more robust approach.