Feature request: Add a "repl" mode to `harper-cli lint`

I have been review tons of old screenshots I have of mainly false positives Harper has made since I've been contributing, to see which still hold true.

But it's fiddly to have to start `harper-cli lint` every time. Would be really nice if there was something like `harper-cli line repl` that just went into a loop prompting for a sentence, printing any lints, then prompting for another sentence, until the user does control-C.

Start by locating the harper-cli lint entry point and reviewing how it currently reads input and prints lints. Run the existing lint command first; done means a repl mode repeatedly prompts for sentences, reports their lints, and continues until Control-C.

Written by the indexing model from the issue text.