Markdown: Don't let Liquid objects interrupt paragraphs - #20085

Description

Since #19838 the Liquid flow construct is allowed to interrupt a paragraph, so a line that merely starts with {{ ... }} is torn out of the paragraph it belongs to:

**ASCII** is a {{glossary("character encoding")}} standard using 7-bit to represent 128
{{glossary("character", "characters")}} used by computers.

becomes two paragraphs with a blank line between them. This is what changed across MDN in v3.9.7.

{{ ... }} is an output expression, it is inline by nature, so it shouldn't end a paragraph any more than `code` does. {% ... %} is a tag and does start a block, which is what #19985 needs, so only that one keeps interrupting. Output for the case above is now byte-identical to 3.9.6 again.

Fixes #20083

Checklist

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