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

docs/README.md

@@ -16,7 +16,7 @@ footer: ISC Licensed | Copyright © 2019-present Starship Contributors
 
 # Used for the description meta tag, for SEO
 metaTitle: "Starship: Cross-Shell Prompt"
-description: Starship is the minimal, blazing fast, and extremely customizable prompt for any shell! Shows the information you need, while staying sleek and minimal. Quick installation available for Bash, Fish, ZSH, Ion, and PowerShell.
+description: Starship is the minimal, blazing fast, and extremely customizable prompt for any shell! Shows the information you need, while staying sleek and minimal. Quick installation available for Bash, Fish, ZSH, Ion, Tcsh, Elvish, Nu, Xonsh, and PowerShell.
 ---
 
 <div class="center">
@@ -106,6 +106,7 @@ description: Starship is the minimal, blazing fast, and extremely customizable p
 
    eval $(starship init ion)
    ```
+
    #### Elvish
 
    ::: warning
@@ -130,7 +131,7 @@ description: Starship is the minimal, blazing fast, and extremely customizable p
    eval `starship init tcsh`
    ```
 
-    #### Nushell
+   #### Nushell
 
    ::: warning
    This will change in the future.
@@ -147,3 +148,13 @@ description: Starship is the minimal, blazing fast, and extremely customizable p
    ]
    prompt = "starship_prompt"
    ```
+
+   #### Xonsh
+
+   Add the following to the end of `~/.xonshrc`:
+
+   ```sh
+   # ~/.xonshrc
+
+   execx($(starship init xonsh))
+   ```