Move charts initialisation further in time

What does this MR do?

Related to #242284 (closed).

This MR fixes the cases when charts are not displayed correctly on non-cached load due to the initialisation happening while the cloaking is hiding the parent container.

The timeline of the events that were causing the issue looks something like this:

Which is also the assumption for the strategy used to fix it: introducing CSSLoaded event that fires when the styles are loaded. To cover for the case when Startup.CSS is not enabled, DOMContentLoaded is also kept.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team