spanner: spansql.ParseDLL error by parsing String functions DDL

## Client

- go
- `cloud.google.com/go/spanner/spansql v1.76.1`

## Environment

## Code and Dependencies

```sql
CREATE TABLE `Posts` (
`Id` STRING(26) NOT NULL,
`Title` STRING(256) NOT NULL,
`TitleSoundex` STRING(MAX) AS (LOWER(SOUNDEX(Title))),
`TitleSoundex_Tokens` TOKENLIST AS (TOKEN(TitleSoundex)) HIDDEN
) PRIMARY KEY (`Id`)
```

Then parse: `spansql.ParseDDL("filename", test.sql)`

## Expected behavior

Works just like in https://console.cloud.google.com/

## Actual behavior

Error: `panic: 01-Posts.sql:4: got "(", want ")" or ","`