Add function to indent things in blocks for nix-indint-line

Add a function `nix-indent-first-line-in-block` that checks if the
last word on the previous line is one of "let, import, [, =, (, {".
Then it adds two spaces of indent to the one of the previous line.

nix-mode.el

@@ -380,6 +380,23 @@ STRING-TYPE type of string based off of Emacs syntax table types"
                                     (current-indentation)))))
     (when matching-indentation (indent-line-to matching-indentation) t)))
 
+(defun nix-indent-first-line-in-block ()
+  "Indent the first line in a block."
+
+  (let ((matching-indentation (save-excursion
+                                (forward-line -1)
+
+                                ;; Grab the full string of the line before the one we're indenting
+                                (let ((line (buffer-substring-no-properties (line-beginning-position) (line-end-position))))
+                                  (if (or (string-match "let$" line)
+                                          (string-match "import$" line)
+                                          (string-match "\\[$" line)
+                                          (string-match "=$" line)
+                                          (string-match "\($" line)
+                                          (string-match "\{$" line))
+                                      (current-indentation))))))
+    (when matching-indentation (indent-line-to (+ 2 matching-indentation)) t)))
+
 (defun nix-mode-make-regexp (parts)
   "Combine the regexps into a single or-delimited regexp.
 PARTS a list of regexps"
@@ -514,6 +531,9 @@ PARTS a list of regexps"
                                             ) -1 0)
                                       )))))
 
+            ;; indent line after 'let', 'import', '[', '=', '(', '{'
+            ((nix-indent-first-line-in-block))
+
             ;; dedent '}', ']', ')' 'in'
             ((nix-indent-to-backward-match))