Proposal: per expression intervalstyle

Problem: I have to set IntervalStyle in separate statement to convert
interval type to ISO8601 string.

This isn't well supported by ORMs or similar tools, requiring us to
set it globally (per role or per database).

Problem #2 (exotic): you can't mix two output styles in a single query.

Proposed solution: special case to_char function to accept values accepted
by
SET intervalstyle to 'XYZ'

So:

SELECT to_char(INTERVAL '8 minutes', 'iso_8601')

will act similar to

SET intervalstyle TO 'iso_8601';
SELECT (INTERVAL '8 minutes')::text
RESET interval_style;