Support low effort case sensitivity switching in filters

#### Support low effort case sensitivity switching in filters

As a user, I would like to have case sensitivity only when I need it, through a syntax less cumbersome than an rc override (this is achievable today through `task rc.search.case.sensitive:yes filter command`). This could be implemented in many ways, but an existing pattern in Vim feels like good fit; Vim supports a case-sensitivity mode called ["smartcase"](https://vimdoc.sourceforge.net/htmldoc/options.html#'nosmartcase'). If the user sets their default case sensitivity mode to "smartcase", then, while searching, if a filter includes any upper case letters, the search is case sensitive; otherwise, it's case insensitive. In the context of taskwarrior, users could set default "smartcase" case sensitivity option and override it when necessary with the rc syntax above.