feat: add parent to XMLElement

[drmohundro]

Initial draft for a parent property for XMLElement... to cover #277.

Sample usage from the unit test:

    func testShouldBeAbleToGetParent() {
        XCTAssertNotNil(xml!["root"]["catalog"]["book"][1]["author"].element?.parent)
        XCTAssertEqual(xml!["root"]["catalog"]["book"][1]["author"].element?.parent?.name, "book")
        XCTAssertEqual(xml!["root"]["catalog"]["book"][1]["author"].element?.parent?.attribute(by: "id")?.text, "bk102")
    }

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7226ebc) 74.06% compared to head (0b84d87) 77.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #278      +/-   ##
==========================================
+ Coverage   74.06%   77.90%   +3.84%     
==========================================
  Files          36       36              
  Lines        1997     2195     +198     
==========================================
+ Hits         1479     1710     +231     
+ Misses        518      485      -33     

Files	Coverage Δ
Source/XMLAttribute.swift	100.00% <ø> (ø)
Source/XMLElement.swift	96.61% <100.00%> (+0.11%)	⬆️
Tests/SWXMLHashTests/XMLParsingTests.swift	86.18% <100.00%> (+1.94%)	⬆️

... and 12 files with indirect coverage changes

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