Make it possible to override JAVA_OPTS directly in the extension

[themkat]

If you run Visual Studio Code from a shell, this might seem redundant. If it is run directly from a GUI on the other hand, it might be difficult for some users to be able to set their JAVA_OPTS and have VSCode pick it up. One example is OS X where many environment variables are not sent if run through Spotlight. (yeah, I know it is possible to tweak, but let's make it more user friendly than that!). This PR aims to make it super easy for people to be able to configure their Java options by need. One big example is to combat issues like #120 and fwcd/kotlin-language-server#441 with a custom heap size setting:

It might also help users who want to tweak other settings easily like the garbage collector being used. Computers with beefy CPUs might have owners who want to use the parallel garbage collector to limit heap size at the cost of CPU usage with -XX:+UseParallelGC.

[themkat]

@fwcd , any thoughts here? Based upon the recent activity in the language server issues and in this repo, it seems like there are several people who might get some use from something like this 🙂

[fwcd]

Thanks!