Configure prettier formatting for sandbox code

[leonardehrenfried]

Summary

Last week I was wondering why my sandbox code was not auto-formatted by prettier. It turns out that sandbox code is not formatted at all!

This PR fixes it.

[codecov]

Codecov Report

Attention: Patch coverage is 27.41935% with 45 lines in your changes missing coverage. Please review.

Project coverage is 69.84%. Comparing base (47dece2) to head (c863768).
Report is 8 commits behind head on dev-2.x.

Files with missing lines	Patch %	Lines
...t/siri/updater/azure/AbstractAzureSiriUpdater.java	26.47%	25 Missing ⚠️
...planner/ext/sorlandsbanen/CoachCostCalculator.java	0.00%	11 Missing ⚠️
...opconsolidation/DecorateConsolidatedStopNames.java	50.00%	0 Missing and 3 partials ⚠️
...lanner/ext/parkAndRideApi/ParkAndRideResource.java	0.00%	2 Missing ⚠️
.../opentripplanner/ext/sorlandsbanen/MergePaths.java	0.00%	2 Missing ⚠️
...entripplanner/ext/restapi/model/ApiRouterInfo.java	0.00%	1 Missing ⚠️
...pentripplanner/ext/restapi/resources/IndexAPI.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #6406      +/-   ##
=============================================
+ Coverage      69.79%   69.84%   +0.04%     
- Complexity     18089    18124      +35     
=============================================
  Files           2064     2069       +5     
  Lines          77209    77266      +57     
  Branches        7859     7854       -5     
=============================================
+ Hits           53886    53964      +78     
+ Misses         20566    20546      -20     
+ Partials        2757     2756       -1     

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