The following applies if the frege compiler is run with the `-make`
[flag](https://github.com/Frege/frege/wiki/Compiler-Manpage#make-mode):
Given two dependent frege files. If the `mainSourceDir` property is not
correctly configured, then the frege compiler cannot find the dependent
frege file and will therefore fail the compilation.

The frege compiler starts at the `mainSourceDir` and searches
recursively for dependent modules. E.g. If `mod.Main.fr` imports `other.Dep.fr`
then the frege compilers searches for `mainSourceDir/other/Dep.fr`.

The name implementation is already used by the java plugin and it
leads to conflicts when combining both plugins.

This adds the following common tasks:
- clean
- check
- assemble
- build
- buildConfiguration task rule
- clean task rule

its dependencies

- the `compileFregeTask` has a new optional input called `mainModuleName`. If
it is set, it and only its dependencies will be compiled, otherwise all
`.fr`files in the `mainSourceDir`will be compiled.

- the internal `DependencyFregeTask` is deleted because it was only
used by the vscode plugin and I decided to remove this gradle plugin
as a dependency.

- an example project folder was added to test the plugin manually.