feat(examples): add simplest "counter" realm

[thehowl]

I must have used this as an example a thousand times, but I just realised it doesn't exist as an example.

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.84%. Comparing base (01ee5a9) to head (f984ee1).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2815      +/-   ##
==========================================
- Coverage   60.84%   60.84%   -0.01%     
==========================================
  Files         563      563              
  Lines       75190    75190              
==========================================
- Hits        45753    45747       -6     
- Misses      26068    26073       +5     
- Partials     3369     3370       +1     

Flag	Coverage Δ
contribs/gnodev	60.65% <ø> (ø)
contribs/gnofaucet	14.46% <ø> (ø)
gno.land	67.17% <ø> (ø)
gnovm	65.63% <ø> (ø)
misc/genstd	80.54% <ø> (ø)
misc/logos	19.88% <ø> (ø)
tm2	62.05% <ø> (+0.05%)	⬆️

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[leohhhn]

Add a test as well? It's also common as an example