Introduce word-diff parser

What does this MR do?

Contributes to #16950 (closed)

We want to introduce a support for git diff --word-diff=porcelain format. This format has differences from the default git diff.

Each line in the output can start with:

• - shows that data in this chunk is unchanged
• + - shows that data in this chunk is added
• - - shows that data in this chunk is removed
• ~ - a newline character

The parser converts word-diff format to the collection of Gitlab::Diff::Line object that we use for diff representation.

To make changes relatively small I've splitted the work into multiple MRs. This MR is just a first step to deliver word-diff format support.

New classes defined here are not used anywhere yet.

Screenshots (strongly suggested)

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