Make more kinds of blocks collapse-able in edit mode

**Is your feature request related to a problem? Please describe.**
HackMD already has some support for folding/collapsing blocks in the edit section, specifically headers and style curly brackets. But there seems to be no support for folding blocks of text in:
- Alert section (`:::spoiler`)
- Code blocks (```)
- Quote section (`>`)
- Multi-layer bullet points (`-`)
- Math blocks (`$$`)

They often contain a lot of text, and sometimes (especially `:::spoiler` and code blocks), their content takes up too much space and would be hard to navigate through all these contents before reaching the later part of the notes.

**Describe the solution you'd like**
I want the aforementioned blocks to be foldable, like how in the current version of HackMD, you can fold the whole header / paragraph by clicking the arrow on the left, next to the line numbers.

**Describe alternatives you've considered**
Maybe you can make the part you wanna collapse h6 (`######`), but that wouldn't make that much sense if there are more support for other kinds of blocks.

**Additional context**

I use Google Chrome 128.0.6613.86 (64-bit), Windows 11 Pro 23H2 (OS build 22631.4112)

https://github.com/user-attachments/assets/11aaf1ea-dd88-4c33-b118-1ca1c3884429