LibWeb: Fix inline blocks swallowing trailing whitespace

In #10434 an issue with leading whitespace in new lines after
a <br> element was fixed by checking whether the last fragment
of LineBox is empty.

However, this introduced a regression by which whitespace following
inline elements was swallowed, so `<b>Test</b> 123` would appear
like `Test123`.

By asking specifically if we are handling a forced linebreak
instead of implicity asking for a property that may be shared by
other Node types, we can maintain the correct behavior in regards
to leading whitespace on new lines, as well as trailing whitespace
of inline elements.

Userland/Libraries/LibWeb/Layout/LineBox.h

@@ -26,6 +26,7 @@ public:
     void trim_trailing_whitespace();
 
     bool is_empty_or_ends_in_whitespace() const;
+    bool ends_with_forced_line_break() const;
 
 private:
     friend class BlockContainer;