Since those crates aren't published and we want to eventually publish powersync_core, this adds the powersync_sqlite_nostd crate that contains sqlite3_capi, sqlite3_alloc and sqlite3_nostd sources.

To simplify the build a bit, I've removed the omit_loadable_extension feature. Since we're not using it in the core extension, we can just unconditionally omit those methods.
Additionally, I've removed the loadable feature. Instead, we now use the loadable mechanism by default. Enabling the static feature switches things over to direct calls. This is a bit friendlier on rust analyzer since we don't have to enable features to get the crate to compile.