Performance: Lazy load `markdown.ts`

[umpox]

Description

Goal: Reduce code required to be downloaded before every page can be shown.

Reason:

• Large dependency that is used in a lot of core places
• It might be that we don't actually need this dependency at all anymore (markdown might be handled entirely on the backend now).
• Streamlined main bundle, faster rendering on every page.

Related PR: https://www.google.com/url?q=https://github.com/sourcegraph/sourcegraph/pull/26843%23issue-1039606444&sa=D&source=docs&ust=1659021460225992&usg=AOvVaw2CFLUmaMgZghfL9TuaOsai

A/C

• Lazy loaded this dependency!

[sourcegraph-bot-2]

Heads up @taylorsperry @jasongornall - the "team/frontend-platform" label was applied to this issue.