New Feature: Support Codeblock inside details/summary tag

[HYChou0515]

Code block inside a details/summary tag need extra empty lines to make it work.

my code with empty lines

echo hello

Or it will not generate a code block

my code with empty lines

```shell echo hello ```

However,

@abnerlee in #399 (comment)
it is now supported. Not no empty line inside the HTML Block is allowed.

It would be super great if Typora support such feature.

[abnerlee]

will be covered by #1939