feat: add support for xonsh (#2807)

* feat: add support for xonsh

* xonsh: add STARSHIP_SESSION_KEY

* xonsh: implement STARSHIP_SESSION_KEY in xonsh

* docs: mention tcsh, elvish, and nu in more places

* xonsh: change STARSHIP_SESSION_KEY implementation

See https://github.com/starship/starship/pull/2807#discussion_r667064149

* xonsh: fix jobs implementation

* xonsh: do not silently discard stderr from starship

src/context.rs

@@ -257,6 +257,7 @@ impl<'a> Context<'a> {
             "elvish" => Shell::Elvish,
             "tcsh" => Shell::Tcsh,
             "nu" => Shell::Nu,
+            "xonsh" => Shell::Xonsh,
             _ => Shell::Unknown,
         }
     }
@@ -494,6 +495,7 @@ pub enum Shell {
     Elvish,
     Tcsh,
     Nu,
+    Xonsh,
     Unknown,
 }