Format .nix files

This does not include any automation for the release branch, but
is based on the configuration of https://github.com/NixOS/nix/pull/12349

    pre-commit run -a nixfmt-rfc-style

tests/functional/repl/doc-comments.nix

@@ -6,55 +6,106 @@
     multiply 2 3
     => 6
     ```
-   */
+  */
   multiply = x: y: x * y;
 
-  /**👈 precisely this wide 👉*/
+  /**
+    👈 precisely this wide 👉
+  */
   measurement = x: x;
 
-  floatedIn = /** This also works. */
+  floatedIn =
+    /**
+      This also works.
+    */
     x: y: x;
 
-  compact=/**boom*/x: x;
-
-  # https://github.com/NixOS/rfcs/blob/master/rfcs/0145-doc-strings.md#ambiguous-placement
-  /** Ignore!!! */
-  unambiguous = 
-    /** Very close */
+  compact =
+    /**
+      boom
+    */
     x: x;
 
-  /** Firmly rigid. */
+  # https://github.com/NixOS/rfcs/blob/master/rfcs/0145-doc-strings.md#ambiguous-placement
+  /**
+    Ignore!!!
+  */
+  unambiguous =
+    /**
+      Very close
+    */
+    x: x;
+
+  /**
+    Firmly rigid.
+  */
   constant = true;
 
-  /** Immovably fixed. */
+  /**
+    Immovably fixed.
+  */
   lib.version = "9000";
 
-  /** Unchangeably constant. */
+  /**
+    Unchangeably constant.
+  */
   lib.attr.empty = { };
 
   lib.attr.undocumented = { };
 
-  nonStrict = /** My syntax is not strict, but I'm strict anyway. */ x: x;
-  strict = /** I don't have to be strict, but I am anyway. */ { ... }: null;
+  nonStrict =
+    /**
+      My syntax is not strict, but I'm strict anyway.
+    */
+    x: x;
+  strict =
+    /**
+      I don't have to be strict, but I am anyway.
+    */
+    { ... }: null;
   # Note that pre and post are the same here. I just had to name them somehow.
-  strictPre = /** Here's one way to do this */ a@{ ... }: a;
-  strictPost = /** Here's another way to do this */ { ... }@a: a;
+  strictPre =
+    /**
+      Here's one way to do this
+    */
+    a@{ ... }: a;
+  strictPost =
+    /**
+      Here's another way to do this
+    */
+    { ... }@a: a;
 
   # TODO
 
-  /** You won't see this. */
+  /**
+    You won't see this.
+  */
   curriedArgs =
-    /** A documented function. */
+    /**
+      A documented function.
+    */
     x:
-    /** The function returned by applying once */
+    /**
+      The function returned by applying once
+    */
     y:
-    /** A function body performing summation of two items */
+    /**
+      A function body performing summation of two items
+    */
     x + y;
 
-  /** Documented formals (but you won't see this comment) */
+  /**
+    Documented formals (but you won't see this comment)
+  */
   documentedFormals =
-    /** Finds x */
-    { /** The x attribute */
-      x
-    }: x;
+    /**
+      Finds x
+    */
+    {
+      /**
+        The x attribute
+      */
+      x,
+    }:
+    x;
 }