Add syntax highlighting for shortcut reference links

Shortcut reference links are defined in three of the most common
Markdown implementations:

- [Pandoc](https://pandoc.org/MANUAL.html#extension-shortcut_reference_links)
- [CommonMark](https://spec.commonmark.org/0.29/#shortcut-reference-link)
- [Github Flavored Markdown](https://github.github.com/gfm/#shortcut-reference-link)

Author: gpanders

syntax/markdown.vim

@@ -83,6 +83,7 @@ syn region markdownLinkText matchgroup=markdownLinkTextDelimiter start="!\=\[\%(
 syn region markdownLink matchgroup=markdownLinkDelimiter start="(" end=")" contains=markdownUrl keepend contained
 syn region markdownId matchgroup=markdownIdDelimiter start="\[" end="\]" keepend contained
 syn region markdownAutomaticLink matchgroup=markdownUrlDelimiter start="<\%(\w\+:\|[[:alnum:]_+-]\+@\)\@=" end=">" keepend oneline
+syn region markdownShortcutLink matchgroup=markdownLinkTextDelimiter start="!\=\[\%(\_[^][]*]\%( \=[[(]\)\@!\)\@=" end="\]" contains=@markdownInline,markdownLineStart
 
 let s:concealends = ''
 if has('conceal') && get(g:, 'markdown_syntax_conceal', 1) == 1
@@ -144,6 +145,7 @@ hi def link markdownLinkText              htmlLink
 hi def link markdownIdDeclaration         Typedef
 hi def link markdownId                    Type
 hi def link markdownAutomaticLink         markdownUrl
+hi def link markdownShortcutLink          markdownId
 hi def link markdownUrl                   Float
 hi def link markdownUrlTitle              String
 hi def link markdownIdDelimiter           markdownLinkDelimiter