(core) Add Markdown cell format

Summary:
Text columns can now display their values as Markdown-formatted text
by changing their cell format to "Markdown". A minimal subset of the
Markdown specification is currently supported.

Test Plan: Browser tests.

Reviewers: Spoffy, dsagal

Reviewed By: Spoffy, dsagal

Subscribers: dsagal, Spoffy

Differential Revision: https://phab.getgrist.com/D4326

app/client/components/Forms/styles.ts

@@ -505,7 +505,7 @@ export const cssMarkdownRender = styled('div', `
 export function markdown(obs: BindableValue<string>, ...args: IDomArgs<HTMLDivElement>) {
   return cssMarkdownRender(el => {
     dom.autoDisposeElem(el, subscribeBindable(obs, val => {
-      el.innerHTML = sanitizeHTML(marked(val));
+      el.innerHTML = sanitizeHTML(marked(val, {async: false}));
     }));
   }, ...args);
 }