Make it easier for contrib modules to specify additional CSpell flag words

Migrated issue

Reported by: drunken monkey

Related to !191 (merged)

Problem/Motivation

As discussed in #3427068: Fix cspell errors in Search API (pp.), it’s currently not easily possible for contrib modules to add additional flag words for CSpell. They would instead have to provide their own copy of .cspell.json, with the risk of it slowly drifting out of sync with the one provided by this module.

This is especially important since the default configuration for contrib modules doesn’t contain the “please” flag word that Core prescribes (see #2921133: Remove "Please" from the codebase), thus making it harder for contrib modules to (reliably) adhere to this Core rule.

Proposed resolution

Add a _CSPELL_FLAGWORDS variable that lets contrib modules specifiy additional flag words to add to the CSpell configuration.

Remaining tasks

• Decide on whether to implement the proposed resolution.
• Implement it.