cylc_lang: permit preprocessing in intercycle offsets

[oliver-sanders]

The lexer didn't permit preprocessing inside of inter-cycle offsets.

This caused the examples in cylc/cylc-flow#6349 the be flagged as errors (which would have caused cylc-doc build failures).

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 (example of preproc in intercycle offset now in the docs)
• Changelog entry included - no changelog for this project
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix - no bugfix release branch for this project

[oliver-sanders]

This gets run through the lexer when the cylc-sphinx-extensions docs are built. Before this change, this code would have caused a build error.

Try yourself with make html.

[oliver-sanders]

CI error for Python "3" is caused by an incompatibility with the slide builder (hieroglyph) and not this PR.

Sadly, hieroglyph is no longer maintained and we have dropped build support in cylc-doc. I expect we will probably strip it out completely (cylc-doc & cylc-sphinx-extensions) in the future as it doesn't look like the project is going to make a comeback :(

[wxtim]

I've had a decent go at breaking this.