Format with fourmolu (#3)

Add the formatting config (similar to default but with indent 2)

Reviewed-on: #3
Co-authored-by: Léana 江 <leana.jiang+git@icloud.com>
Co-committed-by: Léana 江 <leana.jiang+git@icloud.com>

src/Types.hs

@@ -105,10 +105,10 @@ data DocMarkup mod id
     -}
     DocMathInline String
   | {- | Math multiline display
-    \[
+      \[
       mathematical expression
       in multiple lines
-    \]
+      \]
     -}
     DocMathDisplay String
   | {- | Anchors, no spaces allowed