Runtime: [Api Suggestion] Add System.Text.Xml

[JustNrik]

Just as we got System.Text.Json, we should have a high-performance low-allocation XmlSerializer, which could support as well XPath, Linq2Xml and such. It could even allow non-closing tags for HTML parsing

[Emzi0767]

It could even allow non-closing tags for HTML parsing

HTML and XML are different subsets of SGML. An HTML parser is therefore a completely different tool than an XML parser.

[Symbai]

_Offtopic: This is exactly what I always think about when seeing how much the XmlSerializer costs in performance but I was afraid to ask since I think it will get denied. Whether it's a new low-allocation API or just performance improvements to the existing, I'd appreciate any improvements to it. I've even downloaded this repo and tried to make pull requests but I have no idea how to test my changes and verify everything is correct so I'm better leaving this up to someone understanding how it works._

[tarekgh]

@buyaa-n @krwq I triaged this one, please update the tagging if needed.

[GSPP]

Is it possible to fix XmlSerializer instead?

A lot of code depends on its API. Although the API is not perfect I think XmlSerializer is actually a pretty reasonable type to use. It could use some modernization, though.