Source value mapped more than once inspection for @ValueMapping

Nobody has claimed this yet.

For value mappings a source value can only be mapped once. For example
```
import org.mapstruct.Mapper;
import org.mapstruct.ValueMapping;

@Mapper
public interface ValueMapper {

enum A {
A,B
}

enum B {
A,B
}

@ValueMapping(source = "A", target = "B")
@ValueMapping(source = "A", target = "A")
A from(B b);
}
```
is invalide as source `A` is mapped twice. MapStruct report this as

> Source value mapping: "A" cannot be mapped more than once.

Currently the plugin does not recognize this as an error. An inspection should be added for this.

For normal mappings there is already an inspection implemented and could be used as an example: https://github.com/mapstruct/mapstruct-idea/blob/main/src/main/java/org/mapstruct/intellij/inspection/TargetPropertyMappedMoreThanOnceInspection.java.

Start with src/main/java/org/mapstruct/intellij/inspection/TargetPropertyMappedMoreThanOnceInspection.java, which provides the existing inspection pattern. Trace how it detects duplicate normal mappings, then apply the same approach to @ValueMapping source values. Done means duplicate source values are recognized and reported with the expected error message.

Written by the indexing model from the issue text.