Respect XML document encoding

[rgrunber]

An XML document can have a document encoding :

<?xml version="1.0" encoding="UTF-8"?>

We should detect (entirely on client-side) the type of encoding and set it appropriately. VS Code currently doesn't handle this, so maybe we can support it on our end for now.

[ds1709]

I agree. It's very annoying to reopen non UTF-8 xml file every time. Encodimg must be readen from xml file header.

[angelozerr]

I fear that vscode doesn't provide an API to update encoding according to this issue microsoft/vscode#824

vscode editorconfig extension cannot support charset because it is blocked by this API which doesn't exists editorconfig/editorconfig-vscode#35