all: tell the user if their configlet version is outdated

I'd like configlet to tell the user if their configlet version is out of date.

Implementation sketch:
- Store a file locally that contains a timestamp. For example on Linux in `.cache/exercism/configlet/last_update_check.txt` or similar.
- Running a configlet command should check that local timestamp. If more than e.g. 1 day has elapsed, and the `--offline` flag was not provided, configlet makes a request to determine the latest version (perhaps hitting https://raw.githubusercontent.com/exercism/configlet/main/configlet.version, or the GitHub API) .
- If there is an update, advise the user to run `fetch_configlet` (or `configlet upgrade`, if we restore that functionality).

Possible HTTP helper modules:
1. [std/httpclient](https://nim-lang.org/docs/httpclient.html), with `-d:ssl`
2. [treeform/puppy](https://github.com/treeform/puppy), without `-d:ssl`. This uses `libcurl` on macOS/Linux, and the `win32 WinHttp` API on Windows.

There might be some complications regarding static linking, but we have to think about this for the online `configlet lint` checks too (Exercism-wide UUID checking, and checking that a `forked_from` value is valid).

---

This issue has been in the back of my mind for a while - I think there was some discussion about it for `configlet-v2`. But I was reminded of it by today's call, where updating the Exercism CLI was mentioned.

Start by locating the configlet command entry points and the existing --offline handling, then review how online configlet lint checks are planned. Done means a locally cached update-check timestamp limits version requests, --offline prevents the request, and an outdated version advises running fetch_configlet or configlet upgrade.

Written by the indexing model from the issue text.