config: validate task messages and outputs

[oliver-sanders]

At this moment users can do things like this:

[runtime]
  [[task]]
    [[[outputs]]]
      succeeded = failed  # shadowing builtin output, undefined behaviour
      a:b = abc  # not compatible with graph syntax
      required = required  # conflicts with cylc terminology
      BE $$ EF ££ WELL )(*& TON =  ?  # just nasty

• Task messages & outputs should be validated to avoid users shadowing built-in messages, outputs or qualifiers.
• Reserve the _cylc prefix save we need it later.

Also reserve the outputs required, optional and all as keywords for future use by cylc set, see cylc set proposal. This proposal isn't approved yet and won't be implemented until Cylc 8.3.0, however, we may as well reserve these keywords now as the proposal will not change their meaning.

I've checked workflows at our end and have found shockingly little usage of custom outputs and no obvious breakages as a result of these changes.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• CHANGES.md entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at remove duplicate index cylc-doc#617.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[hjoliver]

V. nice 👍