Filtering twig breaks sites

Problem/Motivation

When rendering a twig markup field, the module
* first pipes the twig through a HTML text filter
* only then evaluates twig

See code: https://git.drupalcode.org/project/markup_twig/-/blob/8.x-1.x/markup_twi...

This has some bad consequences:

1) The filter does not see dynamically inserted HTML (e.g. you rely on email obfuscation but the email that twig inserts is not processed)
2) The filter may cough as it sees the twig template which in general is invalid HTML
2a) This break may happen unexpected when updating text format filters
3) Twig may break due to some filter results being invalid twig
3a) This break may happen unexpected when adding some new content

I experienced 2) and 3) in #3150347-6: WSOD and Notices

Proposed resolution

Move the check_markup after executing twig in a #post_render callback.