Register SourceCodeWriters as beans

[jarlehansen]

Currently it is difficult to customize how the source code is written, since the KotlinSourceCodeWriter, JavaSourceCodeWriter and GroovySourceCodeWriter is created inline, inside the ProjectGenerationConfiguration. For example:

@Bean
public MainSourceCodeProjectContributor<KotlinTypeDeclaration, KotlinCompilationUnit, KotlinSourceCode> mainKotlinSourceCodeProjectContributor(
		ObjectProvider<MainApplicationTypeCustomizer<?>> mainApplicationTypeCustomizers,
		ObjectProvider<MainCompilationUnitCustomizer<?, ?>> mainCompilationUnitCustomizers,
		ObjectProvider<MainSourceCodeCustomizer<?, ?, ?>> mainSourceCodeCustomizers) {
	return new MainSourceCodeProjectContributor<>(this.description, KotlinSourceCode::new,
			new KotlinSourceCodeWriter(this.description.getLanguage(), this.indentingWriterFactory),
			mainApplicationTypeCustomizers, mainCompilationUnitCustomizers, mainSourceCodeCustomizers);
}

The change in this PR is to simply change the SourceCodeWriters into beans. This has two benefits. 1) It is easy to override if you need something custom when writing the source code, and 2) we can reused the instance for Main and Test.

[mhalbritter]

Thanks @jarlehansen and congratulations on your first contribution!