added display type interfaces for flagging the default display type of tags

src/DisplayTypes/DisplayBlock.php

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+<?php
+
+namespace ByJoby\HTML\DisplayTypes;
+
+interface DisplayBlock
+{
+}

src/DisplayTypes/DisplayContents.php

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+<?php
+
+namespace ByJoby\HTML\DisplayTypes;
+
+interface DisplayContents
+{
+}

src/DisplayTypes/DisplayInline.php

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+<?php
+
+namespace ByJoby\HTML\DisplayTypes;
+
+interface DisplayInline
+{
+}

src/DisplayTypes/DisplayInlineBlock.php

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+<?php
+
+namespace ByJoby\HTML\DisplayTypes;
+
+interface DisplayInlineBlock
+{
+}

src/DisplayTypes/DisplayNone.php

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+<?php
+
+namespace ByJoby\HTML\DisplayTypes;
+
+interface DisplayNone
+{
+}