buildSrc convention plugins

[aSemy]

Reduce the amount of config in build.gradle.kts by creating buildSrc convention plugins. This shouldn't have any functional impact.

Depends on #100

• set up buildSrc
• create convention plugins for Kotlin, Gradle Plugin, and publishing
• starts using the JVM Test Suite plugin. No functional tests are added here - this is just laying groundwork.
• defines a project-local Maven repository for testing publication (I find this much more convenient than publishing to Maven Local, as the repo is more easily accessed and deleted). Run ./gradlew publishAllPublicationsToProjectLocalRepository and look in ./build/maven-project-local to test it.

[aSemy]

LGTM is failing because of Java versions - it looks like it's using jdk8?

[2022-10-25 18:23:53] [build-stdout] [2022-10-25 18:23:53] [autobuild] > There was a failure while executing work items
[2022-10-25 18:23:53] [build-stdout] [2022-10-25 18:23:53] [autobuild]    > A failure occurred while executing org.gradle.plugin.devel.tasks.internal.ValidateAction
[2022-10-25 18:23:53] [build-stdout] [2022-10-25 18:23:53] [autobuild]       > gradle/kotlin/dsl/plugins/_8d667f1efb3570b8d0df88ad0d3b48e4/OrgPluginGroup has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0

The banner says it's shutting down in a few months, so perhaps it's just best to migrate to the replacements

[qoomon]

I've replaced LGTM with CodeQL 876fc5a344e4e9b3e8e10d901c3b7b877372f442