Add liquid syntax linter rule (#44150)

Co-authored-by: Peter Bengtsson <peterbe@github.com>

src/content-linter/style/github-docs.js

@@ -86,6 +86,10 @@ const githubDocsConfig = {
     severity: 'error',
     'partial-markdown-files': false,
   },
+  'liquid-syntax': {
+    severity: 'error',
+    'partial-markdown-files': true,
+  },
   'liquid-if-tags': {
     // LQ114
     severity: 'error',
@@ -118,6 +122,11 @@ export const githubDocsFrontmatterConfig = {
     severity: 'error',
     'partial-markdown-files': false,
   },
+  'frontmatter-liquid-syntax': {
+    // LQ115
+    severity: 'error',
+    'partial-markdown-files': false,
+  },
 }
 
 // Configures rules from the `github/markdownlint-github` repo