Added basic agda support.

modules/lang/agda/README.org

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+#+TITLE: :lang agda
+
+This module adds support for the [[http://wiki.portal.chalmers.se/agda/pmwiki.php][agda]] programming language.
+
+Emacs support is included in the agda release (you can find installation
+instructions [[https://agda.readthedocs.io/en/latest/getting-started/installation.html][here]]). This module attempts to find the location of ~agda2.el~ via
+the ~agda-mode locate~ command that comes with the agda release. Users can set
+this manually by setting the ~+agda2-dir~ variable. 
+

modules/lang/agda/config.el

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+;;; lang/agda/config.el -*- lexical-binding: t; -*-
+
+(defvar +agda-dir (string-remove-suffix "/agda2.el" (shell-command-to-string "agda-mode locate")))
+
+(def-package! agda2
+  :load-path +agda-dir)
+

modules/lang/agda/doctor.el

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+;; -*- lexical-binding: t; no-byte-compile: t; -*-
+;;; lang/agda/doctor.el
+
+(unless (executable-find "agda-mode")
+  (warn! "Couldn't find agda-mode. Agda support won't work"))