Clean up editor settings (#519)

1. Move to per-language prettier formatter throughout. Global wasn't working
for some folks for TypeScript, so I suspect it's best to use the
per-language setting throughout.

2. Merge the adl-vs .editorconfig into the root .editorconfig, giving better
defaults for editors other than VS Code that support .editorconfig.

3. Add ADL, C# and XML to VS Code settings.json

packages/adl-vs/.editorconfig

@@ -1,16 +0,0 @@
-root = true
-
-[*]
-indent_style = space
-indent_size = 2
-end_of_line = lf
-insert_final_newline = true
-charset = utf-8
-
-[*.cs]
-csharp_new_line_before_open_brace = none
-csharp_new_line_before_catch = false
-csharp_new_line_before_else = false
-csharp_new_line_before_finally = false
-csharp_new_line_before_members_in_anonymous_types = false
-csharp_new_line_before_members_in_object_initializers = false