Add Pod reader

Pod ("Plain old documentation") is a markup languaged used principally
to document Perl modules and programs. Since it was originally meant to
be translated pretty directly to man, the semantics are fairly simple.

This Pod reader was developed with reference to the canonical user and
implementer documentation of Pod: https://perldoc.perl.org/perlpod and
https://perldoc.perl.org/perlpodspec.

There are 1490 .pod, .pl, and .pm in the Perl 5.34 distribution found in
/System/Library/Perl on my mac. Of those, this reader dies with a parse
error on 7 of them. All of them seem to be cases where pod commands are
found within a non-colon-prefixed =begin/=end. perlpodspec says I may
treat this as an error.

MANUAL.txt

@@ -269,6 +269,7 @@ header when requesting a document from a URL:
     - `odt` ([OpenOffice text document][ODT])
     - `opml` ([OPML])
     - `org` ([Emacs Org mode])
+    - `pod` (Perl's [Plain Old Documentation])
     - `ris` ([RIS] bibliography)
     - `rtf` ([Rich Text Format])
     - `rst` ([reStructuredText])
@@ -499,6 +500,7 @@ header when requesting a document from a URL:
 [OPML]: http://dev.opml.org/spec2.html
 [OpenDocument]: http://opendocument.xml.org
 [ODT]: https://en.wikipedia.org/wiki/OpenDocument
+[Plain Old Documentation]: https://perldoc.perl.org/perlpod
 [Textile]: https://textile-lang.com
 [MediaWiki markup]: https://www.mediawiki.org/wiki/Help:Formatting
 [DokuWiki markup]: https://www.dokuwiki.org/dokuwiki

pandoc.cabal

@@ -424,6 +424,7 @@ extra-source-files:
                  test/odt/odt/*.odt
                  test/odt/markdown/*.md
                  test/odt/native/*.native
+                 test/pod-reader.pod
 source-repository head
   type:          git
   location:      git://github.com/jgm/pandoc.git
@@ -604,6 +605,7 @@ library
                    Text.Pandoc.Readers.RTF,
                    Text.Pandoc.Readers.Typst,
                    Text.Pandoc.Readers.Djot,
+                   Text.Pandoc.Readers.Pod,
                    Text.Pandoc.Writers,
                    Text.Pandoc.Writers.Native,
                    Text.Pandoc.Writers.DocBook,
@@ -837,6 +839,7 @@ test-suite test-pandoc
                   Tests.Readers.Man
                   Tests.Readers.Mdoc
                   Tests.Readers.FB2
+                  Tests.Readers.Pod
                   Tests.Readers.DokuWiki
                   Tests.Writers.Native
                   Tests.Writers.ConTeXt

src/Text/Pandoc/Readers.hs

@@ -64,6 +64,7 @@ module Text.Pandoc.Readers
   , readRTF
   , readTypst
   , readDjot
+  , readPod
   -- * Miscellaneous
   , getReader
   , getDefaultExtensions
@@ -100,6 +101,7 @@ import Text.Pandoc.Readers.Native
 import Text.Pandoc.Readers.ODT
 import Text.Pandoc.Readers.OPML
 import Text.Pandoc.Readers.Org
+import Text.Pandoc.Readers.Pod
 import Text.Pandoc.Readers.RST
 import Text.Pandoc.Readers.Textile
 import Text.Pandoc.Readers.TikiWiki
@@ -171,6 +173,7 @@ readers = [("native"       , TextReader readNative)
           ,("typst"        , TextReader readTypst)
           ,("djot"         , TextReader readDjot)
           ,("mdoc"         , TextReader readMdoc)
+          ,("pod"          , TextReader readPod)
           ]
 
 -- | Retrieve reader, extensions based on format spec (format+extensions).