Format XSLT files

Extend the list of extensions that Prettier should treat as XML to include
those commonly used by Extensible Stylesheet Language Transformations (XSLT):

https://en.wikipedia.org/wiki/XSLT

Signed-off-by: Roger Sheen <roger@infotexture.net>

Author: infotexture

src/plugin.ts

@@ -151,6 +151,8 @@ const plugin: Plugin = {
         ".xml.dist",
         ".xproj",
         ".xsd",
+        ".xsl",
+        ".xslt",
         ".xspec",
         ".xul",
         ".zcml"