Some dev improvements:
* Add .editorconfig config file https://editorconfig.org/
* move the singleQuote spec into .editorconfig and add old .changesets/*.md to .prettierignore so that we don't incorrectly reformat new changeset files to single quote from the double quote which they can be autogenerated with in github
* .gitignore Ignore emacs chaff files
* Add `yarn format:head` a convenience command to run prettier against just those files in the head commit
* Some mention of `yarn format` in the docs
* Fix some test html closing tags; authoring mistakes, rather than deliberately malformed html — picked up by an explicit `yarn prettier --write '**/*.html'`
* CI: add a prettier GitHub action to format code automatically
* improve GitHub Action config and format some files
* Apply formatting changes
* CI: make the prettier action a standalone action
* Apply formatting changes
* CI: add push as new trigger event
Co-authored-by: Mark-Fenng <Mark-Fenng@users.noreply.github.com>
