Pimp the docs linter (and assorted changes) (#2553)

### Pimp the docs linter (and assorted changes)

 #### `scripts/lint-docs.js`

- Add an optional cache for faster runs
- Add a final report
- Don't return anything from `exec()`
- Cover more files

 #### `scripts/_command.js`

- Look for a "--cache" option

 #### `package.json` scripts

- Added `watch:lint-docs`
- Added `cleanup:lint` to remove the eslint and lint-docs cache files
- Changed `lint:docs` to use the `--cache` option
- Added `test:js` so that we can run the test suite without the linter
- Changed `test` to defer to `test:js`

 #### Actual lint fixes:

- Bad link in a migration guide
- The unicode dashes in the "https://en.wikipedia.org/wiki/Subject–verb–object" are not escaped by marked

### Some more lint-docs pimping

#### `scripts/lint-docs.js`

- some code reorg and cleanup (take a hint from the local coding conventions)
- fix misc bugs
- pass a User-Agent header to the requests
- even nicer reporting

#### `package.json`

- bump the @babel/parser dep to the latest

#### Docs

- tweaks based on lints missed due to previous bugs

### Docs: use the github page for velocity.js, the home page has too many errors.

Co-Authored-By: Isiah Meadows <contact@isiahmeadows.com>

scripts/_command.js

@@ -18,6 +18,7 @@ process.on("unhandledRejection", function (e) {
 
 module.exports = ({exec, watch}) => {
 	const index = process.argv.indexOf("--watch")
+	const useCache = process.argv.indexOf("--cache") >= 0
 	if (index >= 0) {
 		process.argv.splice(index, 1)
 
@@ -29,7 +30,7 @@ module.exports = ({exec, watch}) => {
 
 		watch()
 	} else {
-		Promise.resolve(exec()).then((code) => {
+		Promise.resolve(exec({useCache})).then((code) => {
 			if (code != null) process.exitCode = code
 		})
 	}