- raise a warning when the expression is not pure expression

this is probably last release for PostgreSQL 12 and 13

- metadata and code cleaning

- minor changes

- allow to use custom pg_config

- disallow late pldbgapi2 initialization, fix some possible crashes

- fix another crash related unexpected order of fmgr hook calls

- fix crash in profiler when parameter of EXECUTE is empty string

- fix crash when plpgsql_check is used on anonymous code block

fix possibly crash when a plan is created by plpgsql, then plancache

is invalidated (due sinval), and same plan is created by plpgsql_check
in passive mode.

- fix crash when pragma assert-column is not correctly used