Adding `additional_lines` field to text position

What does this MR do?

First iteration of adding support for multi-line notes. This adds an additional_lines key to the position field to indicate how many lines before/after the referenced line should be highlighted.

This will not function correctly, however, until #213010 (closed) is resolved as part of the frontend work, or the value for additional_lines somehow brought through into that method.

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

Related #211255 (closed)