Add xml support

[pkage]

Pretty much what it says on the tin. Adds loader support for XML if the [xml] extra is installed. If the extra is not installed, a ModuleNotFound error is thrown. Uses xmltodict under the hood for the xml conversion. A simple example:

<section>
	<key>value</key>
</section>

transforms to

{
	"section": {
		"key": "value"
	}
}

All tests pass (100% coverage) across all supported pythohn versions (via ds dev-all). I had to do fiddling about with pyright to get it to actually accept line-level ignore comments—i suspect that may be due to the particular pyright version? Happy to remove for merge.

_{as good a use as any of a sunday afternoon}