Support INTERVAL type

[apstndb]

INTERVAL type is not yet officially released, but it is available in API definition and it can already be used partially.

https://github.com/googleapis/google-cloud-go/blob/8500e9eb4ec11530dce02573a79cec98361e2d7b/spanner/apiv1/spannerpb/type.pb.go#L108-L113

spanner> SELECT CAST("P1Y2M3DT4H5M6.5S" AS INTERVAL);
+------------------+
|                  |
| INTERVAL         |
+------------------+
| P1Y2M3DT4H5M6.5S |
+------------------+
1 rows in set (0.12 msecs)

Currently, memefish can't parse this query because INTERVAL is keyword.

$ go run github.com/cloudspannerecosystem/memefish/tools/parse@latest -mode statement "SELECT CAST("P1Y2M3DT4H5M6.5S" AS INTERVAL);"                
--- Error
syntax error: :1:33: expected token: <ident>, ARRAY, STRUCT, but: INTERVAL
  1|  SELECT CAST(P1Y2M3DT4H5M6.5S AS INTERVAL);
   |                                  ^~~~~~~~
syntax error: :1:42: expected token: <eof>, but: ;
  1|  SELECT CAST(P1Y2M3DT4H5M6.5S AS INTERVAL);
   |                                           ^