sql: creating a udf with a builtin keyword name should fail

Some keyword functions like `substring` and `trim` are not allowed as UDFs in postgres, e.g.,:

```
marcus=# CREATE FUNCTION substring() RETURNS VOID LANGUAGE SQL AS '';
ERROR: 42601: syntax error at or near "("
LINE 1: CREATE FUNCTION substring() RETURNS VOID LANGUAGE SQL AS '';
^
LOCATION: scanner_yyerror, scan.l:1180
```

We currently allow these functions to be created in CRDB, though they can never be executed. We should return a syntax error for compatibility.

Jira issue: CRDB-31880