language_attributes() template tag should be able to output lang and xml:lang attributes together

[ionfish]

The ​XHTML spec states that one should

Use both the lang and xml:lang attributes when specifying the language of an element. The value of the xml:lang attribute takes precedence.

Currently the language_attributes() function will only output one or the other, depending on the html_type option.

One way of dealing with this issue would be to add a parameter to the function allowing theme authors to specify that the template in which it is used is XHTML (since as far as I know there is no way to determine this through checking the WordPress backend).

[ionfish]

Adds a doctype parameter to the language_attributes() function.

[ryan]

(In [6408]) language_attributes() improvements from ionfish. fixes #5393