Add twirl toolchain support

[Sineaggi]

Allows for the overriding of the java version used to invoke the twirl template compiler.

Resulting build.gradle.kts syntax would look like

tasks.compilePlayTwirlTemplates {
    javaLauncher.set(javaToolchains.launcherFor {
        languageVersion.set(JavaLanguageVersion.of(8))
    })
}

Note that if you're using a new version of java already, you can set the version of java used by the scala compiler via

tasks.compileScala {
    javaLauncher.set(javaToolchains.launcherFor {
        languageVersion.set(JavaLanguageVersion.of(8))
    })
}

[Sineaggi]

I've updated the mr to not directly depend on the JavaLauncher class being available, and have moved the code to a new extended TwirlCompile class. The code should now be compatible with the full gradle range currently supported.

[Sineaggi]

So it looks like the integration tests are failing for this reason

Property 'javaLauncher' annotated with @Internal should not be also annotated with @Optional. This behaviour has been deprecated and is scheduled to be removed in Gradle 7.0. See https://docs.gradle.org/6.8/userguide/more_about_tasks.html#sec:up_to_date_checks for more details.

I had fixed a similar issue in #175

[Sineaggi]

I misread, that error was about optional and not nullable. I've removed the @Optional annotation.

[Sineaggi]

@JLLeitschuh any chance somebody at https://github.com/gradle can check a look?

[JLLeitschuh]

Unfortunately, I'm not with Gradle anymore

[Sineaggi]

@JLLeitschuh Sorry for the ping then.

[Sineaggi]

@big-guy hello! Any chance somebody at gradle can review these changes?

[Sineaggi]

So toolchains were added in gradle 6.7, so versions under that would now be unsupported.

Example failure on Gradle 6.6

FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring project ':app'.
> Could not create task ':app:compilePlayTwirlTemplates'.
   > Could not create task of type 'TwirlCompile'.
      > Could not generate a decorated class for type TwirlCompile.
         > org/gradle/jvm/toolchain/JavaLauncher