Introduce MarkdownElement abstract type

It's convenient for dispatch.

Author: tecosaur

stdlib/Markdown/src/Common/Common.jl

@@ -1,5 +1,7 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
+abstract type MarkdownElement end
+
 include("block.jl")
 include("inline.jl")
 

stdlib/Markdown/src/Common/block.jl

@@ -4,7 +4,7 @@
 # Paragraphs
 # ––––––––––
 
-mutable struct Paragraph
+mutable struct Paragraph <: MarkdownElement
     content
 end
 
@@ -39,7 +39,7 @@ end
 # Headers
 # –––––––
 
-mutable struct Header{level}
+mutable struct Header{level} <: MarkdownElement
     text
 end
 
@@ -95,7 +95,7 @@ end
 # Code
 # ––––
 
-mutable struct Code
+mutable struct Code <: MarkdownElement
     language::String
     code::String
 end
@@ -124,7 +124,7 @@ end
 # Footnote
 # --------
 
-mutable struct Footnote
+mutable struct Footnote <: MarkdownElement
     id::String
     text
 end
@@ -159,7 +159,7 @@ end
 # Quotes
 # ––––––
 
-mutable struct BlockQuote
+mutable struct BlockQuote <: MarkdownElement
     content
 end
 
@@ -188,7 +188,7 @@ end
 # Admonitions
 # -----------
 
-mutable struct Admonition
+mutable struct Admonition <: MarkdownElement
     category::String
     title::String
     content::Vector
@@ -246,7 +246,7 @@ end
 # Lists
 # –––––
 
-mutable struct List
+mutable struct List <: MarkdownElement
     items::Vector{Any}
     ordered::Int # `-1` is unordered, `>= 0` is ordered.
     loose::Bool # TODO: Renderers should use this field
@@ -332,7 +332,7 @@ pushitem!(list, buffer) = push!(list.items, parse(String(take!(buffer))).content
 # HorizontalRule
 # ––––––––––––––
 
-mutable struct HorizontalRule
+mutable struct HorizontalRule <: MarkdownElement
 end
 
 function horizontalrule(stream::IO, block::MD)

stdlib/Markdown/src/Common/inline.jl

@@ -4,7 +4,7 @@
 # Emphasis
 # ––––––––
 
-mutable struct Italic
+mutable struct Italic <: MarkdownElement
     text
 end
 
@@ -20,7 +20,7 @@ function underscore_italic(stream::IO, md::MD)
     return result === nothing ? nothing : Italic(parseinline(result, md))
 end
 
-mutable struct Bold
+mutable struct Bold <: MarkdownElement
     text
 end
 
@@ -66,7 +66,7 @@ end
 # Images & Links
 # ––––––––––––––
 
-mutable struct Image
+mutable struct Image <: MarkdownElement
     url::String
     alt::String
 end
@@ -85,7 +85,7 @@ function image(stream::IO, md::MD)
     end
 end
 
-mutable struct Link
+mutable struct Link <: MarkdownElement
     text
     url::String
 end
@@ -156,7 +156,7 @@ end
 # Punctuation
 # –––––––––––
 
-mutable struct LineBreak end
+mutable struct LineBreak <: MarkdownElement end
 
 @trigger '\\' ->
 function linebreak(stream::IO, md::MD)