all: tell the user if their configlet version is outdated

[ee7]

I'd like configlet to tell the user if their configlet version is out of date.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, with -d:ssl
2. 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).

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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.