Extensions: Add Ext_tex_math_gfm constructor to Extension.

[API change]

This handles two GitHub-specific syntaxes for math.

And document in MANUAL.txt.

This is now default for `gfm`, in addition to `tex_math_dollars`.

See #9121.

MANUAL.txt

@@ -3405,6 +3405,7 @@ Emojis are replaced by their names.
 ## Math Input
 
 The extensions [`tex_math_dollars`](#extension-tex_math_dollars),
+[`tex_math_gfm`](#extension-tex_math_gfm),
 [`tex_math_single_backslash`](#extension-tex_math_single_backslash), and
 [`tex_math_double_backslash`](#extension-tex_math_double_backslash)
 are described in the section about Pandoc's Markdown.
@@ -5766,6 +5767,19 @@ Asian wide characters and other characters.
 
 Parses textual emojis like `:smile:` as Unicode emoticons.
 
+#### Extension: `tex_math_gfm` ####
+
+Supports two GitHub-specific formats for math.
+Inline math: ``$`e=mc^2`$``.
+
+Display math:
+
+````
+``` math
+e=mc^2
+```
+````
+
 #### Extension: `tex_math_single_backslash` ####
 
 Causes anything between `\(` and `\)` to be interpreted as inline

src/Text/Pandoc/Extensions.hs

@@ -133,6 +133,7 @@ data Extension =
     | Ext_task_lists          -- ^ Parse certain list items as task list items
     | Ext_table_captions      -- ^ Pandoc-style table captions
     | Ext_tex_math_dollars    -- ^ TeX math between $..$ or $$..$$
+    | Ext_tex_math_gfm        -- ^ Additional TeX math style used in GFM
     | Ext_tex_math_double_backslash  -- ^ TeX math btw \\(..\\) \\[..\\]
     | Ext_tex_math_single_backslash  -- ^ TeX math btw \(..\) \[..\]
     | Ext_wikilinks_title_after_pipe -- ^ Support wikilinks of style
@@ -398,6 +399,7 @@ getDefaultExtensions "gfm"             = extensionsFromList
   , Ext_yaml_metadata_block
   , Ext_footnotes
   , Ext_tex_math_dollars
+  , Ext_tex_math_gfm
   ]
 getDefaultExtensions "commonmark"      = extensionsFromList
                                           [Ext_raw_html]
@@ -551,6 +553,7 @@ getAllExtensions f = universalExtensions <> getAll f
     , Ext_hard_line_breaks
     , Ext_smart
     , Ext_tex_math_dollars
+    , Ext_tex_math_gfm
     , Ext_superscript
     , Ext_subscript
     , Ext_definition_lists