Increase column limit to 1999

[rkistner]

We currently use JSON to store all synced data, with views to present it as a normal table.

Part of this is using triggers to persist updates (insert/update/delete), using the SQLite json_object function to convert the data back to JSON.

The issue is that SQLite limits function calls to 100 arguments by default, and this can be increased to a maximum of 127. Since json_object requires 2 arguments for each value (key + value), this means we can have a maximum of 50 or 63 columns, depending on the value of SQLITE_MAX_FUNCTION_ARG.

The change here works around the issue by building up the json object in chunks of 50 columns, then merge together using a new powersync_json_merge function. This takes the form of powersync_json_merge(json_object('a', 1, 'b', 2), json_object('c', 3, 'd', 4)). This is slightly less efficient than a single json_object call, so we only do this when there are more than 50 columns in a table.

With SQLITE_MAX_FUNCTION_ARG = 100, this would give us a maximum of 100 * 50 = 5000 columns. However, SQLITE_MAX_COLUMN with a default of 2000 is still a limiting factor, so we have a practical limit of 1999 columns (excluding id).