Use string, array, or array of arrays for many `.gitlab-ci.yml` values

Description

We should be more flexible in .gitlab-ci.yml anywhere we accept an array today. We should accept a single value (string) without array syntax if there's only one value. We already support strings or arrays for script, but not other keys like only.

Also, within an array, we should also accept sub-arrays, and just flatten these out as if they were one large array. This is important for making YAML templating easier to use.

Bonus points: We should accept singular and plural forms of keywords. e.g. variable and variables. I know that's not common, but it's very Rails-y, and awesome to not have to worry about plurality, or mismatching plurality. Variables might be fine, but thinking about include vs includes, it's annoying to use a plural when including a single file just because you can include multiple files. I don't think there is any chance of conflict here, so this is pure DX win.

Proposal

string-job:
  script: echo Only one branch
  only: master

.job-partial: &job-partial
  - echo one
  - echo two

complex-job:
  script:
    - *job-partial
    - echo three

Links / references

Documentation blurb

(Write the start of the documentation of this feature here, include:

1. Why should someone use it; what's the underlying problem.
2. What is the solution.
3. How does someone use this

During implementation, this can then be copied and used as a starter for the documentation.)