[Editor] Change folded code text generation

[G-Ambatte]

This PR will resolve #1907.

This PR modifies the text generated when code is folded, using the following rules:

1. Use the first header in the folded section. At this point, a h6 at the top of the fold will be used over a later h1.
2. If no headers exist, use the text of the first non-empty line.
3. If no non-empty lines exist, generate text "Lines {start line}-{end line}"

---

Example output:

Folded:

Brew Source:

# Combat Maneuvers
\page
# Combat Actions
\page
## Movement
\page
## Defense
\page
## Special Targeting
\page
# Footnotes on source rules
\page
### Parry Defence
\page
## Other options/wrinkles
\page
### Weapon Length and Positioning
\page
## Damage-type Maneuvers
\page
## No Quarter <small>Combat manuevers - for fighting</small>
\page





\page

This is a test page.
This is the second line.

\page

This is a test page.
This is the second line.

### Header later on


\page

[G-Ambatte]

As per the comments on #1907, I've added a color attribute to the LESS for the fold-marker, changing it from blue to grey.

In this example, I've folded all of the code, then re-opened one to make sure that the heading (and it's color) is visible.

[calculuschild]

This seems fine to me now. Any objection to merging @G-Ambatte ?

[G-Ambatte]

I've converted to an actual PR instead of a Draft, so it should be ready to merge!

[calculuschild]

:cook: 🤌