[LANG-1795] Fix EnumUtils.getEnumMap throw IllegalStateException when keyFunction produces duplicate keys

[IcoreE]

LANG-1795
EnumUtils.getEnumMap(...) currently collect enum values using Collectors.toMap(...) without a merge function.Because the default Collectors.toMap requires the key to be unique, these methods will throw an IllegalStateException if the provided keyFunction produces duplicate keys.

class EnumUtilsDuplicateKeyTest {

    enum Color {
        RED, ROSE
    }
    @Test
    void testGetEnumMapThrowsOnDuplicateKey() {
        assertThrows(IllegalStateException.class, () -> {
            EnumUtils.getEnumMap(Color.class, c -> c.name().substring(0, 1)); //Throw IllegalStateException
        });
    }
} 

Proposed fix:
Change the collection step to supply a merge function. A compatible, conservative default is to preserve the first-seen value

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[garydgregory]

-1: Please read https://issues.apache.org/jira/browse/LANG-1795?focusedCommentId=18041281&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-18041281