fix: do not process non-grammar keywords

[mr-cal]

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Another grammar-related fix.

This is caused by this line augmenting data for non-grammar keywords.

It was causing errors like this:

Bad snapcraft.yaml content:
- value is not a valid list (in field 'parts.gnome-calculator.meson-parameters')
Full execution log: '/root/.local/state/snapcraft/log/snapcraft-20240429-201308.751985.log'

My solution is not to process grammar for non-grammar keywords.

[mr-cal]

Is snapcraft passing with this?

Yes, I'm testing with this branch here: https://github.com/canonical/snapcraft/pull/4768

I'm not crazy about the path this grammar handling is going (looks like it's getting more complicated instead of less) but I don't think there's much we can do in a bugfix release, so I'm ok with this

Neither am I, this code was already hard to read to begin with...

[mr-cal]

If I rewrote this function, I would simplify it by splitting up the processing of dictionaries, lists, and strings.

[mr-cal]

CI failures unrelated, they are caused by a ruff update