[feature] Smart Case (selective case sensitive searches)

Only having the binary choice between case sensitivity vs case insensitivity isn't always flexible enough. Sometimes case sensitivity is exactly what is needed, but not very often, but switching between them is ungainly. This is where smart case is useful.

Searches are case insensitive if the search term is in all lower case, but case Sensitive if any upper case characters are included, aka "smart case." I'm sure this concept exists in other places too, but I know of it from Vim.

This has been mentioned in prior issues here also: https://github.com/GothenburgBitFactory/taskwarrior/issues/1131