feat: Add predicate for flyspell to enable spell checking

When `flyspell-mode` (or `flyspell-prog-mode`) is turned on in a buffer that's major mode is `git-commit-ts-mode`, the spell checking doesn't work. This commit alleviates that by adding a predicate that. specifies which words should be spell checked. The predicate is registered as a `flyspell-mode-predicate` property of `git-commit-ts-mode`. I think that's it's most important to spell check subject and message body including trailers and breaking changes. However, it's probably best not to spell check types, scopes, nor tokens of trailers. Also removed email addresses and GitHub style mentions (things that starts with @) from words to check.