Translation support for Snowflake is limited

[fh-afrachioni]

Very few Snowflake-specific translations are currently included in dbplyr, leading to breakage when using common features available with other backends:

dbplyr::translate_sql(grepl('pattern', x), con = dbplyr::simulate_snowflake())
#> <SQL> grepl('pattern', `x`)
dbplyr::translate_sql(str_flatten(x), con = dbplyr::simulate_snowflake())
#> Error in `str_flatten()`:
#> ! str_flatten() is not available in this SQL variant
dbplyr::translate_sql(days(x), con = dbplyr::simulate_snowflake())
#> <SQL> days(`x`)

[mgirlich]

This was fixed in #860.