fix: enhance all kinds xpath queries and clean code

[zhengjitf]

Closes #6898.

The root is the following condition statements miss else clause, so that it will cause error when selectedValues is a string.

insomnia/packages/insomnia/src/ui/components/templating/local-template-tags.ts

Lines 724 to 732 in 1d97689

	if (typeof selectedValues === 'string') {
	results = [{ outer: selectedValues, inner: selectedValues }];
	}
	results = (selectedValues as Node[])
	.filter(sv => sv.nodeType === Node.ATTRIBUTE_NODE
	|| sv.nodeType === Node.ELEMENT_NODE
	|| sv.nodeType === Node.TEXT_NODE)
	.map(selectedValue => {

I found similar code in queryXPath util function, so I refactored the code with it, and adjust queryXPath to deal with all kinds xpath queries.