Support readonly option

[Doug Stevenson]

With better-sqlite3, the readonly option behaves as such:

• Constructor fails when combined with an in-memory database

TypeError: In-memory/temporary databases cannot be readonly

• Queries that mutate fail

SqliteError: attempt to write a readonly database

• db.readonly set to true

This library does none of the above yet.

[Pekka Enberg]

Depends on tursodatabase/libsql#344