Make composer installation steps reusable snippets

The comment that I make reference to:
--
We could defo run things based on a variable (name to be determined) as that's easy enough to trigger or not full jobs (via rules) and to check in the very first part of the script for each job.

Example (pseudocode):

variables:
  NEW_VARIABLE: 1

.run-composer-steps: &run-composer-steps
- do the composer things

composer:
  rules:
    - if NEW_VARIABLE == 0
  script
    - *run-composer-steps

cspell (for example):
  # Drop the "needs" part
  script:
    - if NEW_VARIABLE == 1 OR no-asset-from-composer-found
        *run-composer-steps
      endif
   - rest of the job script

That way we could experiment and turn on/off as needed.