[Core] Improve undefined step reporting

[mpkorstanje]

The various ways to execute Cucumber report undefined steps inconsistently.
Partially because different tools have different reporting needs. Nevertheless:

When reporting per step or scenario the suggestion should include snippets for all
undefined steps in that scenario. This applies to:

• JUnit4
• TestNG
• JUnit5
• Teamcity Plugin

When reporting per test run the suggestion should include snippets for all
undefined steps in the execution. This applies to:

• CLI

When printing snippets they should be copy-pasted into an IDE without further
editing. This means individual snippets or groups of snippets should not be
separated by spacers, text or anything else.

Fixes: #2024

[codecov]

Codecov Report

Merging #2208 (558a17d) into main (e0ad566) will increase coverage by 0.14%.
The diff coverage is 77.31%.

@@             Coverage Diff              @@
##               main    #2208      +/-   ##
============================================
+ Coverage     82.79%   82.94%   +0.14%     
+ Complexity     2340     2334       -6     
============================================
  Files           306      306              
  Lines          8319     8312       -7     
  Branches        764      767       +3     
============================================
+ Hits           6888     6894       +6     
+ Misses         1120     1112       -8     
+ Partials        311      306       -5     

Impacted Files	Coverage Δ	Complexity Δ
.../cucumber/core/runtime/TestCaseResultObserver.java	45.23% <0.00%> (+11.41%)	4.00 <0.00> (ø)
.../cucumber/plugin/event/SnippetsSuggestedEvent.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../src/main/java/io/cucumber/core/runner/Runner.java	97.80% <80.00%> (-1.03%)	29.00 <1.00> (+1.00)	⬇️
...n/java/io/cucumber/core/plugin/TeamCityPlugin.java	83.06% <81.48%> (+0.19%)	45.00 <5.00> (-1.00)	⬆️
...io/cucumber/core/plugin/DefaultSummaryPrinter.java	79.06% <100.00%> (ø)	11.00 <1.00> (ø)
.../junit/platform/engine/UndefinedStepException.java	100.00% <100.00%> (+21.42%)	5.00 <4.00> (-1.00)	⬆️
...src/main/java/io/cucumber/junit/JUnitReporter.java	90.90% <100.00%> (-4.82%)	22.00 <2.00> (ø)
...java/io/cucumber/junit/UndefinedStepException.java	100.00% <100.00%> (+6.06%)	5.00 <5.00> (-4.00)	⬆️
...ava/io/cucumber/testng/UndefinedStepException.java	100.00% <100.00%> (+20.00%)	6.00 <4.00> (-1.00)	⬆️

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