LibWeb: Rename the LayoutMode enum values and explain them

The old mode names, while mechanically accurate, didn't really reflect
their relationship to the CSS specifications.

This patch renames them as follows:

    Default => Normal
    AllPossibleLineBreaks => MinContent
    OnlyRequiredLineBreaks => MaxContent

There's also now an explainer comment with the LayoutMode enum about the
specific implications of layout in each mode.

Userland/Libraries/LibWeb/Layout/InlineFormattingContext.cpp

@@ -84,7 +84,7 @@ void InlineFormattingContext::run(Box const&, LayoutMode layout_mode)
 
     auto& containing_block_state = m_state.get_mutable(containing_block());
 
-    if (layout_mode != LayoutMode::Default) {
+    if (layout_mode != LayoutMode::Normal) {
         containing_block_state.content_width = max_line_width;
     }