its dependencies

The replFregeTask has the following new logic:
1. Compiles the specified fregeRepl module (either in the
   `build.gradle` via command line option `--replModule=...`) and all
   its dependencies.
2. Sets up the correct classpath so that dependent modules don't have
to be imported manually. In addition, it solves the shadowing problem by
removing the replModule java and class file from the classpath.
3. It prints one single command to directly start the repl and load the
   specified module.

Bonus: I designed the task so that you can even automate step 3 with
the following bash command: `eval $(./gradlew -q replFrege)`.

- 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.

The `replSource` property specifies the frege source file that you want to
load into the repl. It is excluded in the `fregeCompile` task so that
we don't get two java class files (one from `compileFrege` and one
from the fregeRepl `:l` command) that shadow each other on the classpath.

As a result, we can make interactive changes to the `replSource` file and
use the `:r` reload command to see them.

e.g:

```groovy
dependencies {
    implementation 'org.frege-lang:fregefx:0.8.2-SNAPSHOT'
}
```