Create a `fork` option to prepare toolchain support

[bmarwell]

Before we implement toolchains, I suggest to implement a fork parameter first.

Rationale:

• Forking is needed for toolchains anyway, as groovyc will be called
• Toolchains without forking might be possible, but is not necessary to implement. Loading additional jars is not trivial.
• We can define parameters for the toolchain.

That said, sdkman uses GROOVY_HOME=…. If that doesn't yield anything for fork mode to use, just look for groovyc in the $PATH. If that did not find anything, abort.

Toolchain Support for #43 would just need to read the toolchain variable and use that installDir parameter (or similar) instead of the default $GROOVY_HOME.

[bmarwell]

Related: see <fork> for the maven compiler plugin as reference.