fix scheme name resolution, and schema load on WSL (#5327)

This PR fixes the scheme resolution bug outlined in #5326

The approach is as follows:

* In [SchemeManager.cs], find the first scheme parser that actually
  successfully parses the scheme, as opposed to the existing code, which
  finds the first scheme parser which _says it can parse the scheme_, as
  that logic spuriously returns `true` currently. 
* In [XmlSchemeParser.cs] and [JsonParser.cs], ensure that the contents
  of the file are read and the contents passed to XmlDocument.LoadXXX,
  as this fails with an UriException on WSL otherwise.
* Remove `CanParse` as it is superfluous. The check for a valid scheme
  parser should not just check an extension but also if the file exists
  - this is best done by the `ParseScheme` function as it already
  returns null on failure.
* Add `FileExtension` to the interface because we need it lifted now.

Closes #5326

src/tools/ColorTool/ColorTool/SchemeManager.cs

@@ -91,9 +91,8 @@ public static void PrintSchemes()
         public static ColorScheme GetScheme(string schemeName, bool reportErrors = false)
         {
             return GetParsers()
-                .Where(parser => parser.CanParse(schemeName))
                 .Select(parser => parser.ParseScheme(schemeName, reportErrors))
-                .FirstOrDefault();
+                .FirstOrDefault(x => x != null);
         }
 
         public static IEnumerable<ISchemeParser> GetParsers()

src/tools/ColorTool/ColorTool/SchemeParsers/ISchemeParser.cs

@@ -8,7 +8,7 @@ namespace ColorTool.SchemeParsers
     interface ISchemeParser
     {
         string Name { get; }
-        bool CanParse(string schemeName);
+        string FileExtension { get; }
         ColorScheme ParseScheme(string schemeName, bool reportErrors = false);
     }
 }