Use Prettier to lint/format instead of Rubocop (#178)

I use CoC in Vim to integrate with language servers similar to VSCode,
but I've never gotten Solargraph, which incorporates Rubocop, to work.
On the other hand, Prettier has been a pleasant experience to use, and I
love how I've never had to write a single configuration file.

To fix this, this commit:

* replaces Rubocop with Prettier;
* reformats all files to match Prettier's format;
* adds a new step to the GitHub workflow to ensure that all files pass
  the lint step; and
* adds an optional Git hook so that before you push, all files will
  linted

Note that in order to get started with this project, you'll need to have
Node installed. I recommend using `asdf` which can manage not only Node
but also Ruby as well. Then you'll want to run `yarn install`. I've
included this in the Contributing doc as well as in the setup script.

Author: mcmire

.gitattributes

@@ -0,0 +1,4 @@
+/.yarn/**            linguist-vendored
+/.yarn/releases/*    binary
+/.yarn/plugins/**/*  binary
+/.pnp.*              binary linguist-generated

.github/workflows/super_diff.yml

@@ -4,16 +4,36 @@ on:
     branches:
       - main
       - ci-*
-    paths-ignore:
-      - '**.md'
   pull_request:
     types:
       - opened
       - synchronize
-    paths-ignore:
-      - '**.md'
+concurrency:
+  group: build-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
 jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          bundler-cache: true
+      - name: Use Node.js
+        uses: actions/setup-node@v3
+        with:
+          node-version-file: ".nvmrc"
+          cache: "yarn"
+      - name: Install Ruby dependencies
+        run: bin/ci/install
+      - name: Install Yarn dependencies
+        run: yarn --immutable
+      - name: Lint
+        run: yarn lint
   test:
+    needs:
+      - lint
     runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
@@ -42,22 +62,23 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - name: Set up Ruby
-        id: set-up-ruby
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
       - name: Run tests
         run: bin/ci/run-tests
   test_ruby_3:
+    needs:
+      - lint
     runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix:
         ruby:
-          - '3.0'
-          - '3.1'
-          - '3.2'
+          - "3.0"
+          - "3.1"
+          - "3.2"
         rails_appraisal:
           - rails_6_0
           - rails_6_1
@@ -82,7 +103,6 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - name: Set up Ruby
-        id: set-up-ruby
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}

.gitignore

@@ -1,8 +1,19 @@
 .DS_Store
 Gemfile.lock
+gemfiles/.bundle
 gemfiles/*.gemfile.lock
+node_modules
 pkg
 spec/examples.txt
 tmp
 zeus.server.log
 zeus.server-start.log
+
+# Ignore Yarn stuff
+.pnp.*
+.yarn/*
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/sdks
+!.yarn/versions

.nvmrc

@@ -0,0 +1 @@
+18.14.0

.prettierignore

@@ -0,0 +1,4 @@
+.yarn/cache
+.yarn/releases
+gemfiles
+vendor/bundle

.rubocop.yml

@@ -0,0 +1,3 @@
+nodeLinker: node-modules
+
+yarnPath: .yarn/releases/yarn-3.4.1.cjs

.tool-versions

@@ -5,26 +5,26 @@ but here are some quick hits:
 
 ## Basic concepts
 
-* An **object inspector** generates a multi-line textual representation of an object,
+- An **object inspector** generates a multi-line textual representation of an object,
   similar to PrettyPrinter in Ruby or AwesomePrint,
   but more appropriate for showing within a diff.
-* An **operation** represents a difference in one value from another
+- An **operation** represents a difference in one value from another
   in the context of a data structure.
-  That difference can either be an *delete*, *insert*, or *change*.
-* An **operation tree** is the set of all operations between two data structures,
+  That difference can either be an _delete_, _insert_, or _change_.
+- An **operation tree** is the set of all operations between two data structures,
   where each operation represents the difference between an inner element within the structure
   (value for an array or a key/value pair for a hash).
   Since change operations represent elements that have child elements,
   they also have child operations to represent those child elements.
   Those child operations can themselves have children, etc.
   This descendancy is what forms the tree.
-* An **operation tree builder** makes a comparison between two like data structures
+- An **operation tree builder** makes a comparison between two like data structures
   and generates an operation tree to represent the differences.
-* A **diff formatter** takes an operation tree
+- A **diff formatter** takes an operation tree
   and spits out a textual representation of that tree in the form of a conventional diff.
   Each operation may in fact generate more than one line in the final diff
   because the object that is specific to the operation is run through an object inspector.
-* Finally, a **differ** ties everything together
+- Finally, a **differ** ties everything together
   and figures out which operation tree builder and diff formatter to use for a particular pair of values
   (where one value is the "expected" and the other is the "actual").