Add support for linting based on schemas in WebIDE

What does this MR do?

Issue: #218472 (closed)

1. Adds support for linting based on schemas in the Web IDE. Now we can easily add schemas to lint any json or yaml files.
2. Adds support for linting .gitlab-ci.yml files.
3. Updates monaco-editor to 0.20.0 and monaco-editor-webpack-plugin to 1.9.0. (An upgrade was required since in order to get monaco-yaml needed a higher version (0.19.x) of monaco-editor than what we currently had (0.18.x).)

Known Issues

When closing the .gitlab-ci.yml file, there is an error in the console:

This is a known issue and is an issue in the monaco-yaml library. I've raised a PR in the upstream repo to fix it. The console error doesn't cause visible issues in the Web IDE though.

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