RFR: JDK-8246435 Replace Javascript implementation of pandoc filters with Java

[Magnus Ihse Bursie]

Due to the removal of Nashorn, the scripts that process pandoc json AST 
files need to be rewritten in Java.

This patch replaces the javascript transformation filters with the same 
functionality in Java. I have verified using the compare script that the 
output is indeed unchanged. I have also reverted the change in 
JDK-8243109 which was a workaround for bootcycle images, which stopped 
working when Nashorn was removed from the JDK. (Since it's still present 
in the Boot JDK, everything except bootcycle images still work fine for 
JDK 15.)

The JSON parser comes from Project Skara. I have made a few patches, 
which cleans up some code (to stop IntelliJ from complaining about 
ugliness), and added some functionality that was needed by this use 
case. For those who are interested, the patchset compared to the 
upstream Skara code is here: 
http://cr.openjdk.java.net/~ihse/skara-json-patches/webrev.01/. I will 
talk with Project Skara to see if they are interested in incorporating 
this patch, so the code bases do not diverge.

Bug: https://bugs.openjdk.java.net/browse/JDK-8246435
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8246435-pandoc-filters-in-java/webrev.01

/Magnus