[Core] Flush pretty output manually

[mpkorstanje]

🤔 What's changed?

For the sake of efficiency and performance we write to buffered writers.
However, the pretty plugin is expected to print scenarios as they're started
and steps as they're finished.

This was initially resolved by flushing the NiceAppendable on every write.
This turned out to have performance impact (#2481) and interact poorly with
tools that expected (#2536) us to be better behaved.

By having NiceAppendable flush only when the underlying buffer was full these
problems were resolved (#2541) . However, because the underlying buffer is 8KiB
the pretty formatters output would effectively sit in a buffer until the writer
was closed.

So now we flush the pretty and progress output manually after each event. This
strikes a balance somewhere between the flush-everything-all-the-time and
flush-when-full approaches.

Fixes: #2563

🏷️ What kind of change is this?

• 🐛 Bug fix (non-breaking change which fixes a defect)

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

[codecov]

Codecov Report

Merging #2573 (2eef85d) into main (0ee15f0) will increase coverage by 0.13%.
The diff coverage is 92.30%.

@@             Coverage Diff              @@
##               main    #2573      +/-   ##
============================================
+ Coverage     84.58%   84.72%   +0.13%     
+ Complexity     2664     2655       -9     
============================================
  Files           310      310              
  Lines          9357     9317      -40     
  Branches        901      899       -2     
============================================
- Hits           7915     7894      -21     
+ Misses         1111     1094      -17     
+ Partials        331      329       -2     

Impacted Files	Coverage Δ
...main/java/io/cucumber/core/plugin/AnsiEscapes.java	95.65% <ø> (+7.65%)	⬆️
...java/io/cucumber/core/plugin/MessageFormatter.java	84.61% <ø> (-1.10%)	⬇️
.../java/io/cucumber/core/plugin/PrettyFormatter.java	85.23% <83.33%> (-0.29%)	⬇️
...ava/io/cucumber/core/plugin/ProgressFormatter.java	90.24% <100.00%> (+0.24%)	⬆️
...n/java/io/cucumber/core/plugin/RerunFormatter.java	100.00% <100.00%> (ø)
.../java/io/cucumber/core/plugin/UTF8PrintWriter.java	100.00% <100.00%> (ø)
...ucumber/core/plugin/UnusedStepsSummaryPrinter.java	86.66% <100.00%> (ø)

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