Add support for external escape valve (^dir)

This commit makes it possible to force nu to treat a command as an external command by prefixing it with `^`. For example `^dir` will force `dir` to run an external command, even if `dir` is also a registered nu command.

This ensures that users don't need to leave nu just because we happened to use a command they need.

This commit adds a new token type for external commands, which, among other things, makes it pretty straight forward to syntax highlight external commands uniquely, and generally to treat them as special.

src/shell/helper.rs

@@ -135,6 +135,10 @@ fn paint_token_node(token_node: &TokenNode, line: &str) -> String {
             item: RawToken::Bare,
             ..
         }) => Color::Green.normal().paint(token_node.span().slice(line)),
+        TokenNode::Token(Tagged {
+            item: RawToken::External(..),
+            ..
+        }) => Color::Cyan.bold().paint(token_node.span().slice(line)),
     };
 
     styled.to_string()