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'`

package.json

@@ -41,6 +41,7 @@
     "test:watch": "yarn turbo run test:watch",
     "test:update": "yarn turbo run test:update",
     "format": "yarn prettier --write '**/*.{ts,md}'",
+    "format:head": "git diff --name-only HEAD^ |grep '\\.ts$\\|\\.md$' |xargs yarn prettier --write",
     "dev": "yarn turbo run dev",
     "repl": "cd packages/rrweb && npm run repl",
     "live-stream": "cd packages/rrweb && yarn live-stream",