While writing some Markdown documentation for Rails, I came across an
interesting case where trying to link to an instance method at the start
of a line would instead parse as an H1 heading:

```markdown
#response_body=
```

Expected:

```html
<a href=""><code>#response_body=</code></a>
```

Actual:

```html
<h1>response_body=</h1>
```

According to the CommonMark spec:

> At least one space or tab is required between the # characters and the
> heading’s contents, unless the heading is empty. Note that many
> implementations currently do not require the space. However, the space
> was required by the original ATX implementation, and it helps prevent
> things like the following from being parsed as headings:
>
> Example 64

So while some implementations do not follow this requirement, I believe
RDoc should because it makes it easy to write text similar to Example 64
(which was used in the new test) and it also enables automatically
linking to instance methods at the start of a line.

Previously, any sort of "rich" markup for a definition list's label
would cause the Markdown parser to not recognize a definition list:

```ruby
md = <<~md
`one`
:    This is a definition
md

doc = RDoc::Markdown.parse(md)
doc # => [doc: [para: "<code>one</code>\n: This is a definition"]]
```

This commit tweaks the grammar for Markdown definition lists so that
labels can include "rich" markup such as bold (`**`), code (```), etc:

```ruby
md = <<~md
`one`
:    This is a definition
md

doc = RDoc::Markdown.parse(md)
doc # => [doc: [list: NOTE [item: ["<code>one</code>"]; [para: "This is a definition"]]]]
```

The [PHP Markdown Extra][1] Spec does not seem to specify whether or not
this should be allowed, but it is allowed in the RDoc format:

```ruby
rdoc = <<~rdoc
+code+::
    This is a definition
rdoc

doc = RDoc::Markup.parse(rdoc)
doc # => [doc: [list: NOTE [item: ["+code+"]; [para: "This is a definition"]]]]
```

so accepting this change increases the parity of the two formats.

[1]: https://michelf.ca/projects/php-markdown/extra/#def-list

https://github.com/ruby/rdoc/commit/8f943bbba4

https://github.com/ruby/ruby/actions/runs/3199301563/jobs/5224898228

https://github.com/ruby/rdoc/commit/369e4fa32d60bc00982801a6848efe5338603ac5

https://github.com/ruby/rdoc/commit/512cc55a0e

https://github.com/ruby/rdoc/commit/3b3a583580

https://github.com/ruby/rdoc/commit/333952a62d