fix: keep non-breaking spaces in text

Since the `u` flag was added to the whitespace regex in TextConverter
(7f03acf), `\s` also matches U+00A0, so ` ` and literal non-breaking
spaces were turned into regular spaces. Leave U+00A0 out of the collapse.

EmphasisConverter used trim(), which does not strip U+00A0, so a
non-breaking space at the edge of <em>/<strong> would now end up inside
the delimiters (`**Total:&nbsp;**`), where it stops them from flanking the
text. Strip it there too; like before, it becomes a regular space outside
the delimiters.

Fixes #258

tests/HtmlConverterTest.php

@@ -378,6 +378,14 @@ EOT;
         $this->assertHtmlGivesMarkdown('<code>&lt;p&gt;Some sample HTML&lt;/p&gt;</code>', '`<p>Some sample HTML</p>`');
     }
 
+    public function testNonBreakingSpaces(): void
+    {
+        $this->assertHtmlGivesMarkdown('<p>This is a test and&nbsp;test</p>', "This is a test and\u{00A0}test");
+        $this->assertHtmlGivesMarkdown("<p>12,50\u{00A0}\u{00A0}€ \n total</p>", "12,50\u{00A0}\u{00A0}€ total");
+        $this->assertHtmlGivesMarkdown('<p><strong>Total:&nbsp;</strong>12,50&nbsp;€</p>', "**Total:** 12,50\u{00A0}€");
+        $this->assertHtmlGivesMarkdown('<p>a<em>&nbsp;</em>b</p>', "a\u{00A0}b");
+    }
+
     public function testSetOption(): void
     {
         $markdown = new HtmlConverter();