feat(docs): replace npm@<7 with yarn

closes #1254

Co-authored-by: jay-bulk <jay.bulk@proton.me>
Co-authored-by: Shahar "Dawn" Or <mightyiampresence@gmail.com>
Co-authored-by: Greg Poole <gpoole@users.noreply.github.com>

readme.md

@@ -29,15 +29,15 @@ This can be achieved by:
 
 And both pin/lock to the same version.
 
-# npm@<7
+# Yarn
 
-`npm@<7` does not automatically install `peerDependencies`,
+Yarn does not automatically install `peerDependencies`,
 so if that's what you're using, install them manually.
 Here is an example, but use it only for reference,
 because your decisions regarding version ranges and range specifiers may vary.
 
 ```
-npm install --save-dev \
+yarn add --dev \
   typescript@\* \
   eslint@^8.0.1 \
   eslint-plugin-promise@^6.0.0 \

src/index.test.ts

@@ -367,11 +367,11 @@ test('Exported rule values do not reference eslint-config-standard ones', (t) =>
   }
 })
 
-test('npm install args in readme satisfy peerDeps', async (t) => {
+test('yarn install args in readme satisfy peerDeps', async (t) => {
   const { pkgJson, pkgPath, ourPeerDeps } = await getPkgDetails()
   const readme = (await readFile(resolve(pkgPath, '..', 'readme.md'))).toString()
-  const match = readme.match(/```\n(npm install .*?)```/s)
-  if (match === null) throw new Error()
+  const match = readme.match(/```\n(yarn add .*?)```/s)
+  if (match === null) throw new Error("couldn't find yarn add")
   if (match.length === 0) throw new Error('failed to find code block')
   if (match.length > 2) throw new Error('matched multiple code blocks')
   const installArgRanges = Object.fromEntries(