Builtin support for dbstat table

[FaFre]

After simolus3/sqlite3.dart#237 landed, I added the dbstat table as a builtin table to the sqlparser.

Open question is, shall the inclusion of the dbstat table be controlled by an option (like its the case for json or fts)?

[simolus3]

Thanks! But I think this should be guarded behind an option (we're usually pretty cautious with the functionality we assume, we also expect a fairly old sqlite3 version without a builder option telling us. This helps users on platforms not using sqlite3_flutter_libs).

I think the best way to expose this is to put dbstat into a module and also refactor sqlparser so that modules can add their own default tables. That would also potentially allow us to analyze things like CREATE VIRTUAL TABLE stat USING dbstat(main).