Lint: Better indentation handling

[wxtim]

Problem

Cylc Lint's indentation handling only compares number of [ to indentation for sections.

Now Lint has access to the Spec it should be able to infer the correct indentation for any non-duplicate section. Even then if sections like 'handlers' should be ok because both workflow and task handlers should have the same indentation.

[oliver-sanders]

If we implement the logic to identify invalid sections e.g. [schdulng], then this should ensure the number of ['s is correct, which should ensure the indentation is correct.

[wxtim]

If we implement the logic to identify invalid sections e.g. [schdulng], then this should ensure the number of ['s is correct, which should ensure the indentation is correct.

We already check whether the number of [ matches the indentation, what we don't check is whether key value pairs are indented correctly; Lint will not spot:

[scheduler]
        allow implicit tasks = false  # Eight spaces

[oliver-sanders]

It's doable, but tricky because of Jinja2. E.G. a lot of people will do this:

# the indentation is nicer before processing
{% for x in y %}
    {{ x }}
{% endfor %}

Rather than this:

# the indentation is nicer after processing
{% for x in y %}
{{ x }}
{% endfor %}

[wxtim]

It's doable, but tricky because of Jinja2. E.G. a lot of people will do this:

In the first instance I would suggest that we should test whether indent_size % 4 == 0 - I'm sure I tested this, but it seems to have gone missing.

In the second instance, we can at least say that a given item should not be less indented than X - e.g. initial cycle point should be indented at least 4 spaces.

[wxtim]

#5772 Is probably the best we can do for now?