Creates a connection to a permanent duckdb database
instance in the provided directory (see neon_dir()). This
connection is also cached, so that code which repeatedly calls
[neon_db] will not stall or hang. Only read_only connections
will be cached.
NOTE: [duckdb::duckdb()] can only support a single read-write connection
at a time. The default option of read_only = TRUE allows
multiple connections. [neon_store()] will automatically set this to
FALSE to allow data import.