userdiff: add builtin diff driver for kotlin language.

The xfuncname pattern finds func/class declarations
in diffs to display as a hunk header. The word_regex
pattern finds individual tokens in Kotlin code to generate
appropriate diffs.

This patch adds xfuncname regex and word_regex for Kotlin
language.

Signed-off-by: Jaydeep P Das <jaydeepjd.8914@gmail.com>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4018/kotlin-inline-class

@@ -0,0 +1,5 @@
+value class RIGHT(Args){
+	// a comment
+	// b comment
+	ChangeMe
+}