feat(markdown): set the preview font size apart from the UI font (#1045)

The Markdown preview had a fixed 14px body and notebook Markdown cells inherited the UI font size, so enlarging them meant scaling the whole UI. Languages & Frameworks › Markdown › Preview font size (application-level, 8–72, following the editor font size until set) now drives the preview and notebook Markdown cells; fenced code scales with it, and notebook code cells and outputs follow the editor font.

Fixes #768

Co-authored-by: jaw52 <[email protected]>

src/bindings.ts

@@ -4122,6 +4122,7 @@ export type LanguagesSettings = {
 	prettier?: PrettierSettings,
 	java?: JavaSettings,
 	ruby?: RubySettings,
+	markdown?: MarkdownSettings,
 };
 
 /**  What a freshly opened window should do once its frontend is up. */
@@ -4352,6 +4353,12 @@ export type ManifestDependency = {
 	to: number,
 };
 
+/**  Languages & Frameworks › Markdown. Like IDEA's, it is application-level only. */
+export type MarkdownSettings = {
+	/**  Preview (and notebook Markdown cell) font size in px; unset follows the editor font size. */
+	previewFontSize?: number | null,
+};
+
 export type Markup = {
 	/**  `markdown` or `plaintext`. */
 	kind: string,