Allow parsing date and time types as JS Date

[soedirgo]

PostgREST returns date & time types as a JSON string (and will continue to do so, since there's no native date/time type in JSON).

However, we can modify the JSON parsing such that date & time columns are returned as JS Date for ease of use.

This will require changes in:

• the generated types, since we need to know which columns have a date/time type - currently they're generated as string which is indistinguishable from e.g. text columns
• this client library, since we need to actually transform these columns to Date during JSON parsing

Requested in:

• added option to supply a custom from and to json converter function #233
• Database file generated doesn't output correct type for TIMESTAMP(0) supabase-js#1267
• "supabase gen" should turn timestamptz into a date type postgres-meta#822