Preserve type when writing to ps_crud

[simolus3]

The trigger responsible for writing into ps_crud used to apply the concat function on values in text columns to strip out the JSON subtype (avoiding e.g. SET foo = json_object() to be encoded as a nested JSON object instead of the string "{}").
Unfortunately, that logic was flawed in two ways:

1. concat turns a null input value into an empty string.
2. We've only applied this on text coumns. Since SQLite allows inserting text data into integer / real columns, we should apply this workaround on all columns.

As a solution, this introduces the powersync_strip_subtype function that preserves the type of the input value but will always replace the subtype of the value with 0.