FE grammar util

What does this MR do?

In #1979 (moved), there is a requirement to generate a dynamic series fragment (e.g. "A" or "A and B" or "A, B, and C"). This MR creates a utility function for this.

Since this utility function is more about grammar than string manipulation, it didn't quite fit in any of our existing string modules. I opted to create a new grammar_util module instead.

See how it is used in the feature MR.

What are the relevant issue numbers?

• gitlab-ee#1979

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer