Improve the text find-replace dialog, also setting items to NA?

[rdstern]

@lilyclements I suggest there are 2 items missing.
a) We don't have a good way here to set values as missing, i.e. NA. We actually do, but in our Replace dialog? The one item not yet in that code is to be able to ignore case in the string set into missing.
We could use the facility in the naniar package, but I wonder if our existing code is good enough that we could just add that element. I have an example, of station data from Zambia, that would be good to test.
b) We have nicely added the starts, end, contains commands into the rename dialog. Should they also be added into the Find/Replace dialog?