Add fold and intersperse functions, add bracket overload where we can choose line separator

[DavidGregory084]

This makes it a lot easier to implement formatting for classic function call syntax like f(a, b, c), expanding to:

f(
  a,
  b,
  c
)

There are overloads of fold and intersperse for both Stream and Collection.
We also add a bracket overload that allows to specify the line separator document, providing a choice of replacement document when a layout is flattened e.g.

f( a, b, c ) // Doc.lineOrSpace

and

f(a, b, c) // Doc.lineOrEmpty

[codecov]

Codecov Report

Base: 98.10% // Head: 98.15% // Increases project coverage by +0.04% 🎉

Coverage data is based on head (9b8ea31) compared to base (dea32e0).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##               main       #1      +/-   ##
============================================
+ Coverage     98.10%   98.15%   +0.04%     
- Complexity       44       51       +7     
============================================
  Files             1        1              
  Lines           264      271       +7     
  Branches         63       63              
============================================
+ Hits            259      266       +7     
  Misses            1        1              
  Partials          4        4              

Impacted Files	Coverage Δ
...main/java/com/opencastsoftware/prettier4j/Doc.java	98.15% <100.00%> (+0.04%)	⬆️

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