Use GitHub theme as default theme for white theme in Monaco

What does this MR do?

Issue: #201931 (closed)

Use GitHub theme as default for Web IDE. Note that the theme wont 100% match with the Pygments theme and it is a known issue to document the differences and invite contributions to fix it.

Screenshots

A comparison of the Monaco theme compared to the Pygments theme for a few common languages. For more languages import this repo in GDK locally.

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