bext: load extensions based on the `enableLegacyExtensions` experimental feature

Created by: taras-yemets

Closes https://github.com/sourcegraph/sourcegraph/issues/39042 Closes #40085 (closed)

1. Updates browser extension bundling process to include code-intel extensions to builds for all browsers (we bundled them for only for Firefox previously).
2. Adds enableLegacyExtensions field to GraphQL Site response with a value equal to site configuration enableLegacyExtensions experimental feature value and a fallback to true (will be flipped after Sourcegraph 4.0 release - https://github.com/sourcegraph/sourcegraph/issues/40460).
3. In the browser extension, request enableLegacyExtensions field and, depending on its value, either load extensions from the extensions registry or load the inline code-intel extensions.

Test plan

• tested locally: extensions are loaded with respect to the enableLegacyExtensions experimental feature
• tests pass in CI