Add xmlSelfClosingTags option to disable auto-self-closing empty elements

Adds a new option xmlSelfClosingTags (default: true) that controls whether
empty elements are rendered as <tag/> or <tag></tag>.

When set to false, empty elements preserve explicit open/close tags,
which is useful for Salesforce metadata and other XML formats where
self-closing syntax causes unwanted VCS diffs.

Fixes: #793

Author: fuleinist