BUG: withExtension is very broken, mangling file some names

I was wondering why my `iwq_extended` queue would always be missed by adds, with them instead ending up in `iwq_extende` instead. Yet targeting `iwq_extended_` _would_ put them there... Turns out `rtrim` is either completely broken or working exactly as intended but a very wrong tool for the job:

```js
"extended.md".rtrim(".md") // --> "extende"
"extendeddddddd.md".rtrim(".md") // --> "extende"
"extendedmmd.d.d.....mmmmdddmmmm......".rtrim(".md") // --> "extende"
```

Given that "add to queue" already adds the extension, I have no clue what the `.rtrim(".md") + ".md"` was supposed to achieve. (`rtrim` is also used at least to generate status bar text, so that's not the only problem.)

Search the plugin for the `rtrim(".md")` calls, then trace both queue-add handling and status bar text generation. Reproduce the examples from the issue and verify that queue names and displayed names preserve the intended filename while still handling the `.md` extension correctly.

Written by the indexing model from the issue text.