There is no longer a dedicated "core" project where DEV_ENV would be defined,
instead whichever is the currently selected version automatically becomes the
main project using the main src folder.
The new preprocessor version brings first-party support for fabric, in
particular for automatically fetching mappings loom and for
automatically remapping between MC versions via intermediary mappings.
This will come in handy when updating to 1.15.
It also supports automatic remapping between fabric and forge on the
same MC version (by going mcp<->srg<->mojang<->intermediary<->yarn).
Therefore this commit replaces the previous 1.13.2 (forge) subproject
with a 1.14.4-forge one. Instead of manually providing full mappings for
1.14.4-fabric to 1.13.2-forge and partial (classes) for 1.13.2-forge to
1.12.2-forge, we now only need partial (classes) for 1.12.2-forge to
1.14.4-forge and preprocessor will take care of the forge to fabric
step (in fact, our mapping file for that is currently completely empty).
In an attempt to write an IDE (IntelliJ) plugin for quicker and easier
working with the preprocessor (e.g. immediately mapping single files
from within the IDE, jumping between different versions for the same
file), the preprocessor gradle plugin declaration now requires you to
explicitly specify variables in the common.gradle and the overall
relationship between projects in the build.gradle (latter is required
anyway, so the plugin can know where it should map between forge and
fabric and which subprojects are which versions).
Since that necessitated some changes to the build.gradle file, I took
the opportunity to convert it to Kotlin. As such we now also use
Gradle's newer plugin-block with the pluginManagement-block instead of
manually declaring buildScript (not for the common.gradle though), which
imo is nicer anyway and comes with various advantages (see gradle docs).
There were also some remapping bugs fixed and some new ones
introduced (e.g. manually declared inner class mappings seem to no
longer function properly under certain conditions).
There's also support for remapping Kotlin now. Just saying.
Failure to load the entity tracker usually (not always) results in
`entityTracker` remaining to be `null`, so there has to be an additional
condition for aborting the thenRun-loadEntityTracker-…-recursion.
