Preformatted text without closing ``` not parsed

[mo-rijndael]

According to gemini spec paragraph 5.4.3

Any line whose first three characters are "```" (i.e. three consecutive back ticks with no leading whitespace) are preformatted toggle lines. These lines should NOT be included in the rendered output shown to the user. Instead, these lines toggle the parser between preformatted mode being "on" or "off".

So, that implies, toggles may not be paired, and there could be only one ``` in file. However, Buran doesn't render text from ``` to end of file at all

Steps to reproduce:

1. Create page starting with three backticks line
2. Do not add three backticks at the end
3. Buran will show you a blank page