Don't spellcheck composer.json

The problem here is that bumping one of core's dependencies introduced a new transient dependency on symfony/polyfill-deepclone, and cspell doesn't like the word "deepclone". You could argue that cspell helped us to notice a new dependency was added, but of course that wouldn't happen if the new dependency was a dictionary word. You could also argue that since we're adding deepclone as a dependency, this is a good prompt to tell us we might want to put it in the dictionary, as we might refer to it in other places.

On the other hand, it felt frustrating that this interrupted my workflow, and I'm not sure it was worthwhile.

TL;DR this should be up for discussion rather than just a matter of just excluding this file from cspell to make it easier.

Updated config and dictionaries. LLM assisted here to identify what we could remove. Only two words were removable due to excluding composer.json, the others were already removable. Have used separate commits to highlight which ones are due to this config change.