Gradle support

[Strum355]

Move away from getting users to have to generate a Maven project from a Gradle project with fiddly Groovy scripts. We demand native support 💢

Direction will be dependent on results of RFC 238

[Strum355]

For a first quick pass (without invoking gradle programmatically), consider: https://github.com/sourcegraph/java-langserver/blob/master/src/main/java/com/sourcegraph/langserver/langservice/gradle/Fradle.java

[Strum355]

Went with Gradle Tooling API approach. Appears to work well for both single-project and multi-project gradle configurations 👍

Appears to not work in certain situations e.g. when java plugin is not in use. Oh well

[Strum355]

Closing issue as it "works". Further tracking issues will be created for individual issues encountered as they occur