Follow CommonMark rules for white space in code blocks

[alya]

In d671683, we stripped trailing whitespace in code blocks to address #3998. However, this is inconsistent with CommonMark, and can cause problems for the user, e.g. making diff blocks invalid.

As discussed on CZO, we should follow the CommonMark rules for processing code blocks:

Whitespace must not be stripped from a fenced code block, except from the info string on the opening fence line and up to three spaces immediately preceding the closing fence on the closing fence line.

As a follow-up, we can consider fixing #3998 by offering users the option to strip trailing whitespace before sending a message when we think it would be helpful to do so.

CZO discussion thread

[zulipbot]

Hello @zulip/server-markdown members, this issue was labeled with the "area: markdown" label, so you may want to check it out!