Implement ToSql for StringView

ToSql is implemented for String, but not StringView, in moonbit-community/postgres@0.0.6. The implementation is also missing from the current main branch.

This means callers that already have a string view, for example from parsing or splitting input, cannot bind it directly as a text parameter:

// @client is moonbit-community/postgres/client.
async fn echo(client : @client.Client, value : StringView) -> Unit {
  ignore(client.query_one("SELECT $1::text", params=[value]))
}

The current workaround is params=[value.to_owned()], which requires an intermediate owned String just to bind the parameter.

Could we add ToSql for StringView, accepting the same PostgreSQL types and using the same text encoding behavior as the existing String implementation in client/scalars.mbt? This would let callers pass string views directly.

Related: #2.

Start in client/scalars.mbt by reading the existing ToSql implementation for String and compare its accepted PostgreSQL types and text encoding behavior. Add the corresponding StringView support, then verify the issue's query example can bind a StringView directly without calling to_owned.