Add XSL support

[fbricon]

(Partially?) fixes #55

For some reason there's no autocompletion support in the xsl. @angelozerr do we need to modify the server as well?

[angelozerr]

The use of https://www.w3.org/1999/XSL/Transform don't give you the XML Schema, you have 3 XML Schema version for XSL. If you wish to support validation and completion for xsl, you need the my POC about XSL extension https://github.com/angelozerr/lsp4xml/tree/master/extensions/org.eclipse.lsp4xml.xsl but this https://github.com/angelozerr/lsp4xml/blob/master/extensions/org.eclipse.lsp4xml.xsl/src/main/java/org/eclipse/lsp4xml/xsl/XSLURIResolverExtension.java#L34 should be fixed (I think core should provide a folder where we can deploy some files of lsp4xml).

Today the extesnion doesn't use the version declared in the XML to choose the well XML Schema, I must do it.

[fbricon]

@angelozerr I think xsl support should probably be a core feature as well

[fbricon]

Regardless, I believe this PR is valuable on its own, even without the server components. @NikolasKomonen feel free to give it a try

[angelozerr]

@angelozerr I think xsl support should probably be a core feature as well

Ok, I will dot it. Please note it exists an issur for XSL at eclipse/lemminx#91

[NikolasKomonen]

Works