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>

package-lock.json

@@ -31,9 +31,9 @@
       }
     },
     "@babel/parser": {
-      "version": "7.6.2",
-      "resolved": "https://registry.npmjs.org/@babel/parser/-/parser-7.6.2.tgz",
-      "integrity": "sha512-mdFqWrSPCmikBoaBYMuBulzTIKuXVPtEISFbRRVNwMWpCms/hmE2kRq0bblUHaNRKrjRlmVbx1sDHmjmRgD2Xg==",
+      "version": "7.7.5",
+      "resolved": "https://registry.npmjs.org/@babel/parser/-/parser-7.7.5.tgz",
+      "integrity": "sha512-KNlOe9+/nk4i29g0VXgl8PEXIRms5xKLJeuZ6UptN0fHv+jDiriG+y94X6qAgWTR0h3KaoM1wK5G5h7MHFRSig==",
       "dev": true
     },
     "@nodelib/fs.scandir": {