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'`

.editorconfig

@@ -0,0 +1,14 @@
+root = true
+
+# initialized from https://prettier.io/docs/en/configuration.html#editorconfig
+[*]
+charset = utf-8
+insert_final_newline = true
+end_of_line = lf
+indent_style = space
+indent_size = 2
+max_line_length = 80
+quote_type = single
+
+[.changeset/*.md]
+quote_type = double