yarn format - prettier improvements & add .editorconfig (#1471)

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'`

CONTRIBUTING.md

@@ -31,6 +31,7 @@ clear and has sufficient instructions to be able to reproduce the issue.
 - Run a cobrowsing/mirroring session locally: `yarn live-stream`
 - Test: `yarn test` or `yarn test:watch`
 - Lint: `yarn lint`
+- Rewrite files with prettier: `yarn format` or `yarn format:head`
 
 ## Coding style