Normalise link reference lookups

Ref: https://github.com/erusev/parsedown/pull/104

src/Components/Inlines/Link.php

@@ -79,6 +79,8 @@ final class Link implements Inline
                 $definition = \strtolower($label);
             }
 
+            $definition = \preg_replace('/\s++/', ' ', \trim($definition));
+
             $data = $State->get(DefinitionBook::class)->lookup($definition);
 
             if (! isset($data)) {

tests/data/text_reference.html

@@ -5,4 +5,8 @@
 one</a> defined on 2 lines</p>
 <p><a href="http://example.com">one</a> with a mixed case label and an upper case definition</p>
 <p><a href="http://example.com">one</a> with the a label on the next line</p>
-<p><a href="http://example.com"><code>link</code></a></p>
+<p><a href="http://example.com"><code>link</code></a></p>
+<p>This one has a <a href="/foo">line
+break</a>.</p>
+<p>This one has a <a href="/foo">line
+break</a> with a line-ending space.</p>

tests/data/text_reference.md

@@ -18,4 +18,12 @@ one][website] defined on 2 lines
 [one]
 [1] with the a label on the next line
 
-[`link`][website]
+[`link`][website]
+
+This one has a [line
+break].
+
+This one has a [line
+break] with a line-ending space.
+
+[line break]: /foo