web: lazy-load `@sentry/browser`

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 like @sentry/browser.

Changes

• Added script tag to the app.html that lazy-loads Sentry SDK.
• Removed @sentry/browser imports (apart from TS types) from the web package.

Results

The main bundle is 4.04% smaller.

Notes

Ad-blocking or script-blocking extensions may prevent Sentry SDK from being fetched and initialized correctly. The solution here would be to use a server-side pass-through for event reporting and our CDN for serving the init script. Created an issue to investigate further: https://github.com/sourcegraph/sourcegraph/issues/26981.

Closes https://github.com/sourcegraph/sourcegraph/issues/26840