Correctly highlight links without suffix (#380)

This is a revert of ad853fe. The original changes in ad853fe claimed
to be in service of this snippet:

    Here is some mono space linked text: [`project`](https://github.com/vim-pandoc/vim-pandoc-syntax).
    If I put here more `mono spaced` text, afterwards, the rest of the document is highlited wrong, try it yourself.

This snippet highlights completely fine, even without the changes made
in ad853fe.

On the other hand, this change prevents this snippet from highlighting
the `[anchor]` inside the sentence:

    This is an [anchor] tag that should be auto-linked.

    [anchor]: https://example.com

I would love to revert the original change. Reverting it seems purely
additive to me (does not break old behaviors, adds new behaviors).

Author: jez

syntax/pandoc.vim

@@ -318,7 +318,7 @@ call s:WithConceal('image', 'syn match pandocImageIcon /!\[\@=/ display', 'conce
 " }}}3
 
 " Definitions: {{{3
-syn region pandocReferenceDefinition start="!\=\[\%(\_[^]]*]\%( \=[[(]\)\)\@=" end="\]\%( \=[[(]\)\@=" keepend
+syn region pandocReferenceDefinition start=/\[.\{-}\]:/ end=/\(\n\s*".*"$\|$\)/ keepend
 syn match pandocReferenceDefinitionLabel /\[\zs.\{-}\ze\]:/ contained containedin=pandocReferenceDefinition display
 syn match pandocReferenceDefinitionAddress /:\s*\zs.*/ contained containedin=pandocReferenceDefinition
 syn match pandocReferenceDefinitionTip /\s*".\{-}"/ contained containedin=pandocReferenceDefinition,pandocReferenceDefinitionAddress contains=@Spell,pandocAmpersandEscape