Use indentation settings in dx fmt and vscode extension

Adds the ability to specify an indent string to public autofmt methods - either
a sequence of spaces or a tab character.

Get the indentation style and size from rustfmt for dx fmt, or from the editor
settings for the vscode extension.

Closes #1595

packages/autofmt/tests/wrong/multi-tab.rsx

@@ -0,0 +1,3 @@
+fn app(cx: Scope) -> Element {
+	cx.render(rsx! { div { "hello world" } })
+}