Add support for D2

[SanchayanMaity]

Adds support for D2, a modern diagram scripting language that turns text to diagrams.

See https://d2lang.com/ for documentation on D2.

Closes #57.

[LaurentRDC]

Thank you for your contribution!

Test failures are due to a breaking change in optparse-applicative. I'm working to mitigate this on the master branch. Once this is done, I'll create a new release with your contribution

[SanchayanMaity]

Thank you for your contribution!

Test failures are due to a breaking change in optparse-applicative. I'm working to mitigate this on the master branch. Once this is done, I'll create a new release with your contribution

Thank you.

FWIW, locally I worked with allow-newer being set to true in my cabal config and had the below diff for everything to work and all tests passing.

diff --git a/executable/Main.hs b/executable/Main.hs
index dc52b6d..382f2a6 100644
--- a/executable/Main.hs
+++ b/executable/Main.hs
@@ -1,14 +1,15 @@
 {-# LANGUAGE ApplicativeDo #-}
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE RecordWildCards #-}
 {-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE TypeApplications #-}
 
 module Main where
 
 import Control.Monad (join, msum, void, when)
 import Data.List (intersperse, (\\))
 import Data.Text (unpack)
 import qualified Data.Text.IO as TIO
 import Data.Version (parseVersion, showVersion)
@@ -318,12 +319,12 @@ showManPage = do
   manualPath <- (</> "pandoc-plot-manual.html") <$> getTemporaryDirectory
   TIO.writeFile manualPath $(embedManualHtml)
   openFile ("file:///" <> manualPath)
 
 -- | Use Doc type directly because of newline formatting
 footer' :: P.Doc
 footer' =
   mconcat
-    [ P.text "More information can be found via the manual (pandoc-plot --manual) or the",
+    [ P.pretty @String "More information can be found via the manual (pandoc-plot --manual) or the",
       P.line,
-      P.text "repository README, located at https://github.com/LaurentRDC/pandoc-plot"
+      P.pretty @String "repository README, located at https://github.com/LaurentRDC/pandoc-plot"
     ]