feat(removeUnknownAndDefaults): apply to xml declarations

[SethFalco]

As standalone="no" is the default value in the XML declaration, it can be safely removed. I've separated this into a separate parameter of the removeUnknownsAndDefaults plugin, and is enabled by default.

See: https://www.w3.org/TR/REC-xml/#sec-rmd

Our XML parser doesn't parse the individual properties in the XML declaration, so we just get the full string of all values. Rather than parse it myself, I opted to use a regex to pull out the sole value we're interested in, since it's the only one with a default.

We can revise this in future if we switch library, for example, I know with txml we could do a much cleaner implementation:

> const txml = require('txml')
undefined
> txml.parse(`<?xml version="1.0"?><svg></svg>`)[0]
{ tagName: '?xml', attributes: { version: '1.0' }, children: [] }

The regex would do the following conversions:

<?xml version="1.0" standalone="no"?>
→
<?xml version="1.0"?>

<?xml standalone='no' version='1.0'?>
→
<?xml version='1.0'?>

<?xml encoding="utf-8" standalone="no" version="1.0"?>
→
<?xml encoding="utf-8" version="1.0"?>

Chores

• Clarify in the docs that removing the XML declaration can mess with MIME-type checking.
• Change some types from Array<…> to …[].

Related

• Closes Option to remove standalone="no" from XML declaration #836