Support mod-assign operator `%=`

[kunli2]

fixes #555

[knutwannheden]

All other assignment operators already appear to be supported and covered by tests in https://github.com/openrewrite/rewrite-kotlin/blob/95dcdeab77ec377524518f2df1fe7a9f70a7c3e4/src/test/java/org/openrewrite/kotlin/tree/AssignmentOperationTest.java.

See also https://kotlinlang.org/docs/reference/grammar.html#assignmentAndOperator.