fix(markdown): preview headings in the preview's text colour (#1171)

Preview headings used the editor scheme's Markdown › Heading attribute (meant for the source) since #992; they now use the editor foreground like the rest of the preview.

Fixes #1151

src/features/viewers/markdown/Markdown.module.css

@@ -56,7 +56,7 @@
 .preview :global(h5),
 .preview :global(h6) {
   margin: 1.2em 0 0.6em;
-  color: var(--syn-markup-heading, var(--fg-strong));
+  color: var(--fg-editor, var(--fg-strong));
   line-height: 1.25;
 }
 .preview :global(h1) {