Most editors do support spellchecking and quite often each project has a list of spelling exceptions, a local dictionary that is stored inside the project.
At this moment this is not standardised but .editorconfig would be the best place to document the filename of this exception dictionary (exception word list).
Also another option related to this would be to specify the spelling languages used by the project as some are using US English, some are using UK English or even other languages. Use of same ISO codes as HTML would be recommended for portability (en, en-us, en-gb, ...)
Most spellcheckers do support "dic" files which are nothing else than list of words, each on a newline.
Don't see how ignore lists of spell checkers can be editor independent. Editors all their own implementation and therefore file formats.
And those files are stored far away in users settings directory for the editors that I am aware of.
If we do provide sample config items for these two options editor will be able to implement this feature, but first we need to document them.
At this moment editors I am aware about do have an option to specify this wordlist in their config but that's per-user settings, not per-project setting in most cases.
Still, if we define/document such an option for editorconfig people could start implementing it for different editors. It is a chicken and the egg problem, as I doubt anyone would implement it inside editorconfig if is not documented first.
I believe a locale setting would indirectly satisfy your requirements.
locale = en-us
This would allow editors to pull their locale-specific list of words, in whatever file format they store. An added benefit is that it might potentially be used for other features involving locale.