XPath support

[apb2006]

Description
Better XPath support. The existing commands are broken

• BaseX tools: get current xpath
• BaseX tools: evaluate xpath

Current Workarounds
Other extensions all have issues with descendant axis

• Feature request: Support xpath evaluation on xml documents redhat-developer/vscode-xml#473
• descendant::, descendant-or-self::, // not working DeltaXML/vscode-xslt-tokenizer#134

Maybe replace broken tree view with something like below, but using fontoxpath

• https://github.com/uli-weltersbach/XPathTools/wiki