Switch to new pandoc-types and use Text instead of String [API change].

PR #5884.

+ Use pandoc-types 1.20 and texmath 0.12.
+ Text is now used instead of String, with a few exceptions.
+ In the MediaBag module, some of the types using Strings
  were switched to use FilePath instead (not Text).
+ In the Parsing module, new parsers `manyChar`, `many1Char`,
  `manyTillChar`, `many1TillChar`, `many1Till`, `manyUntil`,
  `mantyUntilChar` have been added: these are like their
  unsuffixed counterparts but pack some or all of their output.
+ `glob` in Text.Pandoc.Class still takes String since it seems
  to be intended as an interface to Glob, which uses strings.
  It seems to be used only once in the package, in the EPUB writer,
  so that is not hard to change.

stack.yaml

@@ -12,9 +12,8 @@ flags:
 packages:
 - '.'
 extra-deps:
-- pandoc-citeproc-0.16.3.1
-#- pandoc-types-1.17.6.1
-- texmath-0.11.3
+- pandoc-types-1.20
+- texmath-0.12
 - haddock-library-1.8.0
 - skylighting-0.8.2.3
 - skylighting-core-0.8.2.3
@@ -23,8 +22,9 @@ extra-deps:
 - HsYAML-0.2.0.0
 - HsYAML-aeson-0.2.0.0
 - doctemplates-0.7
-- git: https://github.com/jgm/pandoc-types
-  commit: 00f7bb79e79d7cfd3523880dbc64ba3ea46c3da2
+# - pandoc-citeproc-0.16.3.1
+- git: https://github.com/jgm/pandoc-citeproc
+  commit: dc09b028d6876df81cd76b731e58886f77f269b1
 ghc-options:
    "$locals": -fhide-source-paths -Wno-missing-home-modules
 resolver: lts-14.6