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>

docs/migration-v02x.md

@@ -834,7 +834,7 @@ The first of the two headers, `Content-Type`, will trigger a CORS prefetch as it
 
 ## `m.deferred` removed
 
-v0.2.x used its own custom asynchronous contract object, exposed as `m.deferred`, which was used as the basis for `m.request`. v2.x uses Promises instead, and implements a [polyfill](promises.md) in non-supporting environments. In situations where you would have used `m.deferred`, you should use Promises instead.
+v0.2.x used its own custom asynchronous contract object, exposed as `m.deferred`, which was used as the basis for `m.request`. v2.x uses Promises instead, and implements a [polyfill](promise.md) in non-supporting environments. In situations where you would have used `m.deferred`, you should use Promises instead.
 
 ### v0.2.x