Move whitespace formatting to generic XHTML format

[haszi]

Move whitespace formatting from Render.php to the generic XHTML format and add the option for each format to control their own whitespace formatting. Add a test to ensure XHTML formatting is correct.

The current way of handling this in Render.php not only applies to all formats (XHTML, PDF, Manpage, etc.), it also does not allow formats to control the output of significant whitespace. With this PR, formats can implement the format_whitespace() method and override default whitespace output.

This PR also moves the existing conditional whitespace formatting to the generic XHTML format as I assumed this was specifically added for rendering the documentation in XHTML format.

Note:
this also corrects a minor inconvenience in formatting where whitespace after the very first <methodsynopsis>, <constructorsynopsis> or <destructorsynopsis> is printed. This was due to a non-existent $elementStack[$currentDepth] in in_array($elementStack[$currentDepth] ?? "", ["methodname", "methodparam", "type", "void"], true).

Because of this fix, existing tests will need a minor update if this PR is merged which I will take care of.

[haszi]

A note relating to the TODO in the generic XHTML format:

whitespace in <type> elements is not removed because XMLReader returns it as part of the <type>'s text. I'm planning on removing the lines that don't work (right under the TODO) and if there are no objections, add trim the text in the method handling the <type> elements' text.

[Girgias]

Thank you this looks good :)