web: lazy-load code-insights components and `highlight.js`

Created by: valerybugakov

Context

To make our bundle smaller and reduce the initial loading time of the web application, we can lazy-load heavy dependencies and optional components.

Changes

• highlight.js modules specific to different languages are now lazy-loaded.
• code-insights components are lazy-loaded to avoid including d3.js, react-draggable-grid, visx, and src/enterprise/insights into the main bundle.

Results

The main bundle is 18.93% smaller. 132kb gzipped size was cut from the main bundle.

Before	After

Further improvements

• https://github.com/sourcegraph/sourcegraph/issues/26841 — 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
• https://github.com/sourcegraph/sourcegraph/issues/26840 — Lazy-loading Sentry would yield another ~86kb gzip cut in the main bundle size.
• Splitting site-admin/backend.ts into multiple modules would allow to lazy-load parts of it, but the benefits won't be as noticeable: ~5kb gzip.

Closes https://github.com/sourcegraph/sourcegraph/issues/26433.