SpellCheckCustomDictionary

### WebKittens @annevk @marcoscaceres ### Title of the proposal SpellCheckCustomDictionary ### URL to the spec https://github.com/Igalia/explainers/blob/main/spell-check-dictionary/README.md ### URL to the spec's repository https://github.com/Igalia/explainers/ ### Issue Tracker URL _No response_ ### Explainer URL _No response_ ### TAG Design Review URL https://github.com/w3ctag/design-reviews/issues/1191 ### Mozilla standards-positions issue URL https://github.com/mozilla/standards-positions/issues/1384 ### WebKit Bugzilla URL _No response_ ### Radar URL _No response_ ### Description @ziransun has a prototype in chromium for Igalia, work sponsored by Bloomberg Tech. We want to get a good sense of positions from WebKit and Mozilla.