Move prettier file globs into .prettierrc.js file.

This allows us to DRY up the `package.json` file and allows editors which
understand Prettier support to be aware of exactly which files are meant to
be covered, rather than including it within the CLI flags.

.prettierignore

@@ -1,3 +1,5 @@
 *.json
 *.md
 *.snap
+
+dist/

.prettierrc.js

@@ -1,4 +1,26 @@
 module.exports = {
-  trailingComma: 'all',
-  singleQuote: true,
+  // Unfortunately, prettierrc doesn't support explicitly enumerating the files
+  // we wish to "prettify", instead relying on them being passed as a glob on
+  // the CLI.  See https://github.com/prettier/prettier/issues/3764.
+  //
+  // Unfortunately, that complicates the `package.json` scripts since it
+  // requires duplicating globs in multiple places, and also prevents
+  // Prettier-enabled editors from knowing what files are to be covered.
+  //
+  // We can DRY this up a bit by leveraging "requirePragma", an instruction
+  // that tells prettier to only prettify files which contain `@prettier`
+  // (which none of the files in this repository have) and then specifying the
+  // exact files to be prettified.  As the issue above notes, this should become
+  // more succinct in Prettier 2.x.
+  requirePragma: true,
+  overrides: [
+    {
+      files: '{docs/{,source/**},.,packages/**,test}/{*.js,*.ts}',
+      options: {
+        requirePragma: false,
+        trailingComma: 'all',
+        singleQuote: true,
+      },
+    },
+  ],
 };

package.json

@@ -9,8 +9,10 @@
   "scripts": {
     "postinstall": "lerna run prepare",
     "compile": "lerna run compile",
-    "lint": "prettier-check --ignore-path .gitignore \"{docs/{,source/**},.,packages/**,test}/{*.js,*.ts}\"",
-    "lint-fix": "prettier --write --ignore-path .gitignore \"{docs/{,source/**},.,packages/**,test}/{*.js,*.ts}\"",
+
+    "lint": "prettier-check '**/*.{js,ts}'",
+    "lint-fix": "prettier '**/*.{js,ts}' --write",
+
     "test": "jest --verbose",
     "posttest": "npm run lint",
     "testonly": "npm test",