Revert:27bb1b2f194d4a1bb43fb0aa3dacc423d2c0117d remove date_add and date_diff test after Interval is implemented

Author: xd1313113

docs/partiql-lang-kotlin-omitted-syntax-tests.md

@@ -51,9 +51,6 @@ https://github.com/partiql/partiql-lang-kotlin/blob/34625c68dbcbaf7b8ae60df7a4cf
   https://github.com/partiql/partiql-lang-kotlin/blob/34625c68dbcbaf7b8ae60df7a4cf65c60b2a3b79/lang/test/org/partiql/lang/errors/ParserErrorsTest.kt#L883-L895
 - `offsetBeforeLimit` -- not part of spec; postgresql allows; kotlin impl+mysql+sqlite don't allow; included as test for now
 https://github.com/partiql/partiql-lang-kotlin/blob/34625c68dbcbaf7b8ae60df7a4cf65c60b2a3b79/lang/test/org/partiql/lang/errors/ParserErrorsTest.kt#L1221-L1233
-- date_add/date_diff tests - date_add and date_diff aren't part of specs
-  - https://github.com/partiql/partiql-lang-kotlin/blob/34625c68dbcbaf7b8ae60df7a4cf65c60b2a3b79/lang/test/org/partiql/lang/syntax/SqlParserTest.kt#L957-L1036
-  - https://github.com/partiql/partiql-lang-kotlin/blob/34625c68dbcbaf7b8ae60df7a4cf65c60b2a3b79/lang/test/org/partiql/lang/errors/ParserErrorsTest.kt#L1743-L1858
 - `countExpressionStar`
 https://github.com/partiql/partiql-lang-kotlin/blob/34625c68dbcbaf7b8ae60df7a4cf65c60b2a3b79/lang/test/org/partiql/lang/errors/ParserErrorsTest.kt#L1966-L1978
 - `*` and path `*` with other select list items