Use dedicated properties file for buildscript

[WaitingIdly]

changes in this PR:

• create a new file, buildscript.properties, and move all gradle.properties only used in the build.gradle file into the new file.
  • load the options from this file in build.gradle.
  • the entries not moved are org.gradle.logging.stacktrace and org.gradle.jvmargs.
• add comments describing the files and how to use them.
  • also comment two entries missing a comment, modName and modGroup.
  • adjust the comment for the modId, as it incorrectly stated that the it was "convention" to be all lowercase instead of a requirement, see FMLModContainer#sanityCheckModId throwing IllegalArgumentException, preventing it a mod from being loaded in ModContainerFactory#build.
• adjust README.md and info/error messages that referenced the gradle.properties file to instead indicate the buildscript.properties file.

reasoning:

• separates out the custom properties that arent used anywhere other than build.gradle into their own dedicated file, ensuring they dont pollute the global configs.
• reduces possible confusion involving custom gradle properties (what settings are buildscript vs custom?) and associated clutter.
  • this is particularly relevant with GroovyScript, Universal Tweaks, and other mods like them.

potential concerns:

• migrations are not needed for this change:
  • if the buildscript.properties file isnt found, and single error will be noted, but loading will continue.
  • as the entries from gradle.properties are loaded in every file, them not being moved doesnt cause any issues.
  • furthermore, actually making a migration to do this is problematic - converting the existing gradle.properties file is non-viable due to breaking custom gradle properties.
• the "jump to source/uses" feature of intellij might not work properly. for me, it appears to only identify the source uses when they are inside a string.

PR review considerations:

• adding instructions for how to migrate in order to get the benefit of this might be worthwhile.
• it might be better to have a different name for buildscript.properties- ie build.properties.
• the readme likely needs improvement.

[ALongStringOfNumbers]

This now also could use a line about configuring gradle.properties.