encoding/xml: add indent to xml.Comment

[Faetu]

Add indent before and after xml.Comment

With this change the comment is added with indent at the start, like StartElement and indent at the end, like EndElement.
Fixes #56340

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[Faetu]

This change adds xml.Comment's with indent but you are still able to add xml.Comment's in the same line as the xml.EndElement like before. You have to set indent to empty string:

encoder.Indent("", "")
encoder.EncodeToken(xml.Comment("Comment text"))
encoder.Indent("", "    ")

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Patch Set 1:

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Patch Set 2:

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