Add Acronym practice exercise

[kephas]

It's missing for #48in24 week 7.

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[kahgoh]

I noticed the commit is missing a number of changes:

• the track's config.json is missing the change to add the exercise
• the exercise is missing the .meta directory and its example.lfe, config.json and tests.toml
• the exercise is also missing .docs directory and its instructions.md file (and possible introduction.md if it exists)

These files can be obtained using configlet's sync command.

[kephas]

I pushed a new version according to the reviews.

[ErikSchierboom]

@kephas I can't re-open this PR. Could you recreate the PR and ping me once you've done that?