Handle xml:space attribute & preserve whitespace

[thekid]

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

In editing XML documents, it is often convenient to use "white space" (spaces, tabs, and blank lines) to set apart the markup for greater readability. Such white space is typically not intended for inclusion in the delivered version of the document. On the other hand, "significant" white space that should be preserved in the delivered version is common, for example in poetry and source code.
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When declared, it must be given as an enumerated type whose values are one or both of "default" and "preserve"

[thekid]

Released in https://github.com/xp-forge/address/releases/tag/v5.4.0