[Markdown] Add YAML frontmatter highlighting

Markdown/Markdown.sublime-syntax

@@ -180,7 +180,23 @@ variables:
     #     \s*$      # any amount of whitespace until EOL
     #   )
 contexts:
+  file-start:
+    - match: (---)\s*
+      captures:
+        0: meta.frontmatter.markdown
+        1: punctuation.section.block.begin.frontmatter.yaml.markdown
+      embed: scope:source.yaml
+      embed_scope: meta.frontmatter.markdown source.yaml.embedded
+      escape: ^(---)\s*$  # pandoc requires the remainder of the line to be blank
+      escape_captures:
+        1: meta.frontmatter.markdown punctuation.section.block.end.frontmatter.yaml.markdown
+
   main:
+    - include: file-start
+    - match: ''
+      set: markdown
+
+  markdown:
     - match: |-
         (?x)^
         (?=  {{block_quote}}

Markdown/MultiMarkdown.sublime-syntax

@@ -1,6 +1,7 @@
 %YAML 1.2
 ---
 # http://www.sublimetext.com/docs/3/syntax.html
+version: 2
 name: MultiMarkdown
 first_line_match: (?i)^format:\s*complete\s*$
 scope: text.html.markdown.multimarkdown
@@ -25,4 +26,12 @@ contexts:
     - match: ''
       push:
         - meta_scope: meta.content.multimarkdown
-        - include: scope:text.html.markdown
+        - match: ''
+          embed: scope:text.html.markdown
+          escape: $(?!$)  # should never match
+    # Replace with the following once
+    # https://github.com/sublimehq/sublime_text/issues/2395#issuecomment-662515815 is fixed.
+    # - match: ''
+    #   embed: scope:text.html.markdown
+    #   embed_scope: meta.content.multimarkdown
+    #   escape: $(?!$)  # should never match

Markdown/syntax_test_markdown.md

@@ -289,6 +289,12 @@ Paragraph break.
 
 Paragraph break.
 
+---
+|^^^ meta.block-level meta.separator.thematic-break
+|^^ punctuation.definition.thematic-break
+
+Paragraph break.
+
 --------
 |^^^^^^^^ meta.block-level meta.separator.thematic-break
 |^^^^^^^ punctuation.definition.thematic-break