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'`

.prettierignore

@@ -0,0 +1,107 @@
+# list of old changeset files that were mutated to use single quotes by a previous version of prettier:
+.changeset/attribute-text-reductions.md
+.changeset/avoid-costly-createlement.md
+.changeset/brave-numbers-joke.md
+.changeset/breezy-cats-heal.md
+.changeset/breezy-mice-breathe.md
+.changeset/calm-bulldogs-speak.md
+.changeset/calm-oranges-sin.md
+.changeset/chatty-cherries-train.md
+.changeset/clean-plants-play.md
+.changeset/clean-shrimps-lay.md
+.changeset/cold-eyes-hunt.md
+.changeset/cold-hounds-teach.md
+.changeset/controller-finish-flag.md
+.changeset/cool-grapes-hug.md
+.changeset/cuddly-readers-warn.md
+.changeset/curvy-apples-lay.md
+.changeset/curvy-balloons-brake.md
+.changeset/date-now-guard.md
+.changeset/dirty-rules-dress.md
+.changeset/eight-terms-hunt.md
+.changeset/empty-bikes-cheer.md
+.changeset/event-single-wrap.md
+.changeset/fair-dragons-greet.md
+.changeset/fast-chefs-smell.md
+.changeset/few-rockets-travel.md
+.changeset/few-turkeys-reflect.md
+.changeset/five-peas-lay.md
+.changeset/fluffy-planes-retire.md
+.changeset/forty-elephants-attack.md
+.changeset/fresh-cars-impress.md
+.changeset/fresh-spoons-drive.md
+.changeset/friendly-numbers-leave.md
+.changeset/gold-apples-joke.md
+.changeset/gold-terms-look.md
+.changeset/grumpy-ways-own.md
+.changeset/hip-worms-relax.md
+.changeset/hungry-dodos-taste.md
+.changeset/itchy-dryers-double.md
+.changeset/khaki-dots-bathe.md
+.changeset/large-ants-prove.md
+.changeset/lazy-squids-draw.md
+.changeset/lazy-toes-confess.md
+.changeset/lemon-lamps-switch.md
+.changeset/light-fireants-exercise.md
+.changeset/little-radios-thank.md
+.changeset/little-suits-leave.md
+.changeset/loud-seals-raise.md
+.changeset/lovely-pears-cross.md
+.changeset/lovely-students-boil.md
+.changeset/mean-tips-impress.md
+.changeset/mighty-ads-worry.md
+.changeset/mighty-bulldogs-begin.md
+.changeset/mighty-frogs-sparkle.md
+.changeset/modern-doors-watch.md
+.changeset/moody-dots-refuse.md
+.changeset/nervous-buses-pump.md
+.changeset/nervous-kiwis-nail.md
+.changeset/nervous-mirrors-perform.md
+.changeset/nervous-poets-grin.md
+.changeset/nervous-tables-travel.md
+.changeset/new-snakes-call.md
+.changeset/nice-pugs-reply.md
+.changeset/old-dryers-hide.md
+.changeset/polite-olives-wave.md
+.changeset/pretty-plums-rescue.md
+.changeset/pretty-schools-remember.md
+.changeset/proud-experts-jam.md
+.changeset/rare-adults-sneeze.md
+.changeset/README.md
+.changeset/real-masks-explode.md
+.changeset/real-trains-switch.md
+.changeset/rich-crews-protect.md
+.changeset/rich-dots-lay.md
+.changeset/rich-jars-remember.md
+.changeset/rotten-spies-enjoy.md
+.changeset/serious-ants-juggle.md
+.changeset/silver-pots-sit.md
+.changeset/silver-windows-float.md
+.changeset/sixty-impalas-laugh.md
+.changeset/small-olives-arrive.md
+.changeset/smart-ears-refuse.md
+.changeset/smart-geckos-cover.md
+.changeset/smooth-papayas-boil.md
+.changeset/smooth-poems-bake.md
+.changeset/spotty-bees-destroy.md
+.changeset/stupid-ghosts-help.md
+.changeset/swift-dancers-rest.md
+.changeset/swift-peas-film.md
+.changeset/thin-vans-applaud.md
+.changeset/thirty-baboons-punch.md
+.changeset/three-baboons-bow.md
+.changeset/tidy-swans-repair.md
+.changeset/tidy-yaks-joke.md
+.changeset/tiny-buckets-love.md
+.changeset/tiny-candles-whisper.md
+.changeset/tiny-chairs-build.md
+.changeset/tricky-panthers-guess.md
+.changeset/twenty-goats-kneel.md
+.changeset/twenty-lies-switch.md
+.changeset/twenty-planets-repeat.md
+.changeset/violet-melons-itch.md
+.changeset/violet-zebras-cry.md
+.changeset/wise-spiders-jog.md
+.changeset/witty-kids-talk.md
+.changeset/yellow-mails-cheat.md
+.changeset/young-timers-grow.md