feat: Add rule checking Prism languages

The linter now checks that fences have languages and that those languages are
supported by PrismJS. The linter has been extended over the guide with its own
set of rules that only validate code fences.

tools/lint/lint.test.js

@@ -0,0 +1,57 @@
+/* global describe it expect jest beforeEach */
+const path = require('path');
+
+const lint = require('./lint');
+
+describe('markdown linter', () => {
+  let good = { path: path.join(__dirname, './fixtures/good.md') };
+  let badYML = { path: path.join(__dirname, './fixtures/badYML.md') };
+  let badFencing = { path: path.join(__dirname, './fixtures/badFencing.md') };
+  beforeEach(() => {
+    console.log = jest.fn();
+    // the linter signals that a file failed by setting
+    // exitCode to 1, so it needs (re)setting to 0
+    process.exitCode = 0;
+  });
+  afterEach(() => {
+    process.exitCode = 0;
+  });
+
+  it('should pass `good` markdown', done => {
+    function callback() {
+      expect(process.exitCode).toBe(0);
+      done();
+    }
+    lint(good, callback);
+  });
+
+  it('should fail invalid YML blocks', done => {
+    function callback() {
+      expect(process.exitCode).toBe(1);
+      done();
+    }
+    lint(badYML, callback);
+  });
+
+  it('should fail when code fences are not surrounded by newlines', done => {
+    function callback() {
+      expect(process.exitCode).toBe(1);
+      done();
+    }
+    lint(badFencing, callback);
+  });
+
+  it('should write to the console describing the problem', done => {
+    function callback() {
+      const expected =
+        // eslint-disable-next-line max-len
+        'fixtures/badYML.md: 19: yaml-linter YAML code blocks must be valid [bad indentation of a mapping entry at line 3, column 17:\n          testString: testString\n                    ^] [Context: "```yml"]';
+      expect(console.log.mock.calls.length).toBe(1);
+      expect(console.log.mock.calls[0][0]).toEqual(
+        expect.stringContaining(expected)
+      );
+      done();
+    }
+    lint(badYML, callback);
+  });
+});