build, doc, tools: add eslint-plugin-markdown

* Install eslint-plugin-markdown@1.0.0-beta.7
* Add doc/.eslintrc.yaml
* Add `plugins: [markdown]` to the main .eslintrc.yaml
* .js files in doc folder added to .eslintignore
* Update Makefile, vcbuild.bat, and tools/jslint.js

Refs: #12563
Refs: #12640
Refs: #14047

PR-URL: #14067
Reviewed-By: James Snell <jasnell@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>

.eslintignore

@@ -7,3 +7,4 @@ test/tmp*/
 tools/eslint
 node_modules
 benchmark/tmp/
+doc/**/*.js

.eslintrc.yaml

@@ -1,5 +1,8 @@
 root: true
 
+plugins:
+  - markdown
+
 env:
   node: true
   es6: true

Makefile

@@ -751,13 +751,13 @@ bench-idle:
 
 jslint:
 	@echo "Running JS linter..."
-	$(NODE) tools/eslint/bin/eslint.js --cache --rulesdir=tools/eslint-rules \
-	  benchmark lib test tools
+	$(NODE) tools/eslint/bin/eslint.js --cache --rulesdir=tools/eslint-rules --ext=.js,.md \
+	  benchmark doc lib test tools
 
 jslint-ci:
 	@echo "Running JS linter..."
 	$(NODE) tools/jslint.js $(PARALLEL_ARGS) -f tap -o test-eslint.tap \
-		benchmark lib test tools
+		benchmark doc lib test tools
 
 CPPLINT_EXCLUDE ?=
 CPPLINT_EXCLUDE += src/node_root_certs.h

doc/.eslintrc.yaml

@@ -0,0 +1,12 @@
+## Docs-specific linter rules
+
+rules:
+  # ease some restrictions in doc examples
+  no-restricted-properties: 0
+  no-undef: 0
+  no-unused-vars: 0
+  strict: 0
+
+  # add new ECMAScript features gradually
+  no-var: 2
+  prefer-const: 2