Add cspell-project-words.txt to fix misspelled words [#3570752]

Problem/Motivation

Fix cspell issues:

src/Form/ConfigForm.php:129:247   - Unknown word (unallowed)  -- for the whole list unallowed attributes.'));
	 Suggestions: [unalloyed, unhallowed, unplowed, unallied, unavowed]
CSpell: Files checked: 10, Issues found: 1 in 1 file.

The number of distinct unrecognised/misspelled words is 1
------------
unallowed
------------

File example:
https://git.drupalcode.org/project/cookies_addons/-/blob/1.0.x/.cspell-p...

Issue fork block_attributes-3570752

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