[ruby/rdoc] Allow rich definition list labels for Markdown

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

ruby/rdoc@8f943bbba4

lib/rdoc/markdown.rb

@@ -16445,12 +16445,12 @@ def _DefinitionListItem
     return _tmp
   end
 
-  # DefinitionListLabel = StrChunk:label @Sp @Newline { label }
+  # DefinitionListLabel = Inline:label @Sp @Newline { label }
   def _DefinitionListLabel
 
     _save = self.pos
     while true # sequence
-      _tmp = apply(:_StrChunk)
+      _tmp = apply(:_Inline)
       label = @result
       unless _tmp
         self.pos = _save
@@ -16777,7 +16777,7 @@ def _DefinitionListDefinition
   Rules[:_TableAlign] = rule_info("TableAlign", "< /:?-+:?/ > @Sp {                 text.start_with?(\":\") ?                 (text.end_with?(\":\") ? :center : :left) :                 (text.end_with?(\":\") ? :right : nil)               }")
   Rules[:_DefinitionList] = rule_info("DefinitionList", "&{ definition_lists? } DefinitionListItem+:list { RDoc::Markup::List.new :NOTE, *list.flatten }")
   Rules[:_DefinitionListItem] = rule_info("DefinitionListItem", "DefinitionListLabel+:label DefinitionListDefinition+:defns { list_items = []                        list_items <<                          RDoc::Markup::ListItem.new(label, defns.shift)                         list_items.concat defns.map { |defn|                          RDoc::Markup::ListItem.new nil, defn                        } unless list_items.empty?                         list_items                      }")
-  Rules[:_DefinitionListLabel] = rule_info("DefinitionListLabel", "StrChunk:label @Sp @Newline { label }")
+  Rules[:_DefinitionListLabel] = rule_info("DefinitionListLabel", "Inline:label @Sp @Newline { label }")
   Rules[:_DefinitionListDefinition] = rule_info("DefinitionListDefinition", "@NonindentSpace \":\" @Space Inlines:a @BlankLine+ { paragraph a }")
   # :startdoc:
 end

test/rdoc/test_rdoc_markdown.rb

@@ -305,6 +305,25 @@ def test_parse_definition_list_multi_line
     assert_equal expected, doc
   end
 
+  def test_parse_definition_list_rich_label
+    doc = parse <<-MD
+`one`
+:    This is a definition
+
+**two**
+:    This is another definition
+    MD
+
+    expected = doc(
+      list(:NOTE,
+        item(%w[<code>one</code>],
+          para("This is a definition")),
+        item(%w[*two*],
+          para("This is another definition"))))
+
+    assert_equal expected, doc
+  end
+
   def test_parse_definition_list_no
     @parser.definition_lists = false