Investigate lazy-loading `markdown.ts`

[valerybugakov]

htmlparser2, postcss, marked are required for the synchronous renderMarkdown function. To make it lazy-loadable a considerable time investment is required. The benefits would be noticeable: ~55kb gzip.
https://github.com/sourcegraph/sourcegraph/blob/6805b81059190e86edcb8a257ca8bec834e81f1c/client/shared/src/util/markdown.ts#L55

[github-actions]

Heads up @umpox @valerybugakov @5h1rU @pdubroy @taylorsperry - the "team/frontend-platform" label was applied to this issue.