feat: support additional doc comment markdown

[adalinesimonian]

Simulates formatting that Godot does not natively support in its limited BBCode implementation, such as lists and footnotes, as best as possible.

Also fixes #811 as paragraphs are correctly formatted, without excessive line breaks.

Markup

/// *documented* ~ **documented** ~ [AABB] < [pr](https://github.com/godot-rust/gdext/pull/748)
/// 
/// This is a paragraph. It has some text in it. It's a paragraph. It's quite
/// long, and wraps multiple lines. It is describing the struct `Player`. Or
/// maybe perhaps it's describing the module. It's hard to say, really. It even
/// has some code in it: `let x = 5;`. And some more code: `let y = 6;`. And a
/// bunch of **bold** and *italic* text with _different_ ways to do it. Don't
/// forget about [links](https://example.com).
///
/// a few tests:
///
/// headings:
///
/// # Some heading
///
/// lists:
///
/// - lists
/// - like this
///   - with sublists  
///     that are multiline
///     - and subsublists
/// - and list items
/// * maybe with `*` as well
/// 
/// [reference-style link][somelink]
///
/// links with back-references:
///
/// Blah blah[^foo] Also same reference[^foo]
/// [^foo]: https://example.org
///
/// footnotes:
///
/// We cannot florbinate the glorb[^florb]
/// [^florb]: because the glorb doesn't flibble.
/// 
/// Third note in order of use[^1] and fourth [^bignote]
/// 
/// [^1]: This is the third footnote in order of definition.
/// [^bignote]: Fourth footnote in order of definition.
/// [^biggernote]: This is the fifth footnote in order of definition.
/// 
/// Fifth note in order of use. [^someothernote]
/// 
/// [^someothernote]: sixth footnote in order of definition.
/// 
/// Sixth footnote in order of use. [^biggernote]
///
/// task lists:
///
/// We must ensure that we've completed
/// - [ ] task 1
/// - [x] task 2
///
/// tables:
///
/// | Header1 | Header2 |
/// |---------|---------|
/// | abc     | def     |
///
/// images:
///
/// ![Image](https://godotengine.org/assets/press/logo_small_color_light.png)
/// 
/// ![Image][image]
///
/// blockquotes:
///
/// > Some cool thing
///
/// ordered list:
///
/// 1. thing one
/// 2. thing two
///     1. thing two point one
///     2. thing two point two
///     3. thing two point three
///
///
/// Something here < this is technically header syntax
/// ---
/// And here
///
/// smart punctuation
///
/// codeblocks:
///
/// ```rust
/// #![no_main]
/// #[link_section=".text"]
/// #[no_mangle]
/// static main: u64 = 0x31c0678b10;
/// ```
/// 
/// ```gdscript
/// extends Node
/// 
/// func _ready():
///    print("Hello, world!")
/// ```
/// 
/// ```csharp
/// using Godot;
/// 
/// public class Player : Node2D
/// {
///     [Export]
///     public float Speed = 400.0f;
/// }
/// ```
///
/// Some HTML to make sure it's properly escaped:
///
/// <br/> <- this is inline HTML
///
/// &lt;br/&gt; <- not considered HTML (manually escaped)
///
/// `inline<br/>code`
///
/// ```html
/// <div>
///   code&nbsp;block
/// </div>
/// ```
///
/// [Google: 2 + 2 < 5](https://www.google.com/search?q=2+%2B+2+<+5)
///
/// connect
/// these
/// 
/// [somelink]: https://example.com
/// [image]: https://godotengine.org/assets/press/logo_small_color_dark.png

Output

[lilizoey]

this is really nice to get done! i think overall it looks pretty good, largely just some style questions here and there. it's a bit unfortunate that the diff is confused here though

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1017

[Bromeon]

Thanks a lot for improving this part, really great work!

I also like how you emulate absent features like superscripts and list bullets 👍

[Bromeon]

Thanks! Upon closer look, the code is of really high quality, and the implementation looks very solid. I mostly pointed out some smaller formatting/readability issues. Could you also run rustfmt?

I'll also try to find some time to test it in action 🙂

[adalinesimonian]

Addressed the review comments and rebased onto the main branch.

[adalinesimonian]

Apparently I borked the ordered list incremention. Whoops.

[adalinesimonian]

Should be good now.