highlighter: support Front Matter for YAML meta data

src/hgmarkdownhighlighter.cpp

@@ -818,7 +818,8 @@ void HGMarkdownHighlighter::parseInternal()
     memcpy(content, data, len);
     content[len] = '\0';
 
-    pmh_markdown_to_elements(content, pmh_EXT_STRIKE, &result);
+    int exts = pmh_EXT_STRIKE | pmh_EXT_FRONTMATTER;
+    pmh_markdown_to_elements(content, exts, &result);
 }
 
 void HGMarkdownHighlighter::handleContentChange(int /* position */, int charsRemoved, int charsAdded)

src/resources/themes/v_detorte/v_detorte.mdhl

@@ -177,3 +177,6 @@ foreground: ccb24c
 STRIKE
 foreground: e57373
 font-style: strikeout
+
+FRONTMATTER
+foreground: af8787

src/resources/themes/v_moonlight/v_moonlight.mdhl

@@ -176,3 +176,6 @@ foreground: 6e7686
 STRIKE
 foreground: e57373
 font-style: strikeout
+
+FRONTMATTER
+foreground: 6e7686

src/resources/themes/v_native/v_native.mdhl

@@ -172,3 +172,6 @@ foreground: 00af00
 STRIKE
 foreground: b71c1c
 font-style: strikeout
+
+FRONTMATTER
+foreground: 6c6c6c

src/resources/themes/v_pure/v_pure.mdhl

@@ -173,3 +173,6 @@ foreground: 00af00
 STRIKE
 foreground: b71c1c
 font-style: strikeout
+
+FRONTMATTER
+foreground: 6c6c6c