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'`

docs/development/coding-style.md

@@ -1,6 +1,6 @@
 # Coding Style
 
-These are the style guidelines for coding in Electron.
+These have been adapted from the style guidelines for coding in Electron.
 
 You can run `yarn lint` to show any style issues detected by `eslint`.
 
@@ -9,6 +9,8 @@ You can run `yarn lint` to show any style issues detected by `eslint`.
 - End files with a newline.
 - Using a plain `return` when returning explicitly at the end of a function.
   - Not `return null`, `return undefined`, `null` or `undefined`
+- run `yarn format` to rewrite all files in the standard format
+- run `yarn format:head` to rewrite files from your last commit
 
 ## Documentation