Try to use xml:lang to set html lang

[austinmatherne-wk]

Reason for change

The default lang value of en-US potentially conflicts with xml:lang if present.

Description of change

First check if there's an xml:lang value to use before falling back to en-US when setting lang.

Steps to Test

Note: this doesn't impact the stub viewer, so this must be tested from the command line.

• Use a report with a non English xml:lang value.
• python arelleCmdLine.py --plugins ixbrl-viewer --file austin-2024-05-28-fr.zip --save-viewer ixbrlviewer.html
• Load the viewer in a web browser and confirm that the xml:lang and lang attributes on the html element match (fr).

review: @Arelle/arelle @paulwarren-wk