Lint: Warn-if Jinja2 in Cylc comment

[wxtim]

Summary

In Cylc config files Jinja2 in comments is evaluated. This may or may not be what the user wants and is more likely than not to be an unexpected behavior.

Seealso: cylc/cylc-textmate-grammar#36 (review)

Desired behaviour:

#!jinja2
# flow.cylc file
{% set X = 41 %}
# {% set X = 42 %}
...

cylc lint .
WARNING: 4: Jinja2 in "# {% set X = 42 %}" will be evaluated

Pull requests welcome!

[wxtim]

Merged into #4917